<form method="post" id="loginform" name="loginform" ng-submit="loginUser()" novalidate>
    <div>
        {{message}}
    </div>
    <div>
        <label>User Name</label>
        <input type="text" id="txtUserName" ng-model="user.UserName" name="user.UserName" />
    </div>
    <div>
        <label>Password</label>
        <input type="text" id="txtPassword" ng-model="user.Password" name="user.Password" />
    </div>
    <div>
        <input type="submit" id="btnLogin" title="Save" name="btnLogin" value="Login" />
    </div>
</form>

var demoApp = angular.module('demoApp', []);

demoApp.controller("homeController", ["$scope", "$timeout", function ($scope, $timeout) {

    $scope.loginUser = function () {

        var form = document.getElementById("loginform");
        //var form = $scope.loginform; - tried this here...
        //var form = $scope["#loginform"]; tried this
        //var form = angular.element(event.target); - tried this...
        // tried a lot of other combinations as well...

        form.attr("method", "post");
        form.attr("action", "Home/Index");
        form.append("UserName", $scope.user.UserName);
        form.append("Password", $scope.user.Password);
        form.append("RememberMe", false);
        form.submit();
    };
}]);

$scope.loginUser = function () {
  if($scope.loginform.$valid){
      user.rememberme=false;

       $http({
         url: 'Home/Index',
         method: "POST",
         data: user
    })
    .then(function(response) {
        // success
    },
     function(response) { // optional
        // failed
    });

  }
};

<form method="post" id="loginform" name="loginform" ng-submit="loginUser()" novalidate>
<div>
    {{message}}
</div>
<div>
    <label>User Name</label>
    <input type="text" id="txtUserName" required ng-model="user.UserName" name="UserName" />
</div>
<div>
    <label>Password</label>
    <input type="text" id="txtPassword" ng-model="Password" name="user.Password"required />
</div>
<div>
    <input type="submit"  ng-disabled="myForm.UserName.$invalid || myForm.Password.$invalid" id="btnLogin" title="Save" name="btnLogin" value="Login" />
</div>
</form>

<form id="loginform" name="loginform" ng-submit="loginUser()">
    <div>
        {{message}}
    </div>
    <div>
        <label>User Name</label>
        <input type="text" id="txtUserName" ng-model="user.UserName" name="user.UserName" required/>
        <div class="help-block" ng-messages="loginform.txtUserName.$error" ng-show="loginform.txtUserName.$touched">
            <p ng-message="required">Username is required.</p>
        </div>
    </div>
    <div>
        <label>Password</label>
        <input type="text" id="txtPassword" ng-model="user.Password" name="user.Password" required/>
        <div class="help-block" ng-messages="loginform.txtPassword.$error" ng-show="loginform.txtPassword.$touched">
            <p ng-message="required">Password is required.</p>
        </div>
    </div>
    <div>
        <input type="submit" id="btnLogin" title="Save" name="btnLogin" value="Login" ng-click="loginUser()" />
    </div>
</form>

var demoApp = angular.module('demoApp', ['ngMessages']);

demoApp.controller("homeController", ["$scope", "$timeout", function ($scope, $timeout) {

    $scope.loginUser = function () {
      if($scope.loginform.$valid){
        //Code to run before submitting (but not validation checks)
      } else{
         return false;
     }
    };
}]);

<div ng-controller="homeController" ng-init="construct()">
    <form method="post" action="Index" role="form" id="loginform" name="loginform" ng-form-commit novalidate class="ng-pristine ng-invalid ng-invalid-required">
        <div class="form-group">
            <label for="UserName">User ID</label>
            <input autocomplete="off" class="form-control ng-valid ng-touched ng-pristine ng-untouched ng-not-empty"
                   id="UserName" name="UserName" ng-model="user.UserName" type="text" value=""
                   ng-change="userNameValidation = user.UserName.length == 0">
            <span class="field-validation-error text-danger" ng-show="userNameValidation">The User ID field is required.</span>
        </div>

        <div class="form-group">
            <label for="Password">Password</label>
            <input autocomplete="off" class="form-control ng-valid ng-touched ng-pristine ng-untouched ng-not-empty"
                   id="Password" name="Password" ng-model="user.Password" type="password" value=""
                   ng-change="passwordValidation = user.Password.length == 0">
            <span class="field-validation-error text-danger" ng-show="passwordValidation">The Password field is required.</span>
        </div>

        <div>
            <input type="button" id="btnLogin" title="Login" name="btnLogin" value="Login" ng-click="validateUser(loginform)" />
        </div>
    </form>
</div>

var demoApp = angular.module('demoApp', []);

demoApp.factory("commonService", function () {
    return {
        isNullOrEmptyOrUndefined: function (value) {
            return !value;
        }
    };
});

//This is the directive that helps posting the form back...
demoApp.directive("ngFormCommit", [function () {
    return {
        require: "form",
        link: function ($scope, $el, $attr, $form) {
            $form.commit = function () {
                $el[0].submit();
            };
        }
    };
}]);

demoApp.controller("homeController", ["$scope", "commonService", function ($scope, commonService) {

    $scope.construct = function construct() {
        $scope.user = { UserName: "", Password: "" };
    };

    $scope.userNameValidation = false;
    $scope.passwordValidation = false;
    $scope.isFormValid = false;

    $scope.validateUser = function ($form) {
        $scope.isFormValid = true;

        $scope.userNameValidation = commonService.isNullOrEmptyOrUndefined($scope.user.UserName);
        $scope.passwordValidation = commonService.isNullOrEmptyOrUndefined($scope.user.Password);

        $scope.isFormValid = !($scope.userNameValidation || $scope.passwordValidation);

        if ($scope.isFormValid === true) {
            $scope.loginUser($form);
        }
    };

    $scope.loginUser = function ($form) {
        $form.commit();
    };
}]);

(function(angular) {
  'use strict';

  function DemoFormCtrl($timeout, $sce) {
    var ctrl = this;
    this.$onInit = function() {
      this.url = $sce.trustAsResourceUrl(this.url);
      /*$timeout(function() {
        ctrl.form.$$element[0].submit();
      });*/
    };

    this.validate = function(ev) {
      console.log('Running validation.');
      if (!this.form) {
        return false;
      }
    };
  }

  angular.module('app', [])
    .component('demoForm', {
      template: `
      <p>To run this demo allow pop-ups from https://plnkr.co</p>
      <hr>
      <p>AngularJS - submit form programmatically after validation</p>
      <form name="$ctrl.form" method="get" target="blank" action="{{::$ctrl.url}}" novalidate
       ng-submit="$ctrl.validate($event)">
        <input type='hidden' name='q' ng-value='::$ctrl.value'>
        <input type='hidden' name='oq' ng-value='::$ctrl.value'>
        <input type="submit" value="submit...">
      </form>`,
      controller: DemoFormCtrl,
      bindings: {
        url: '<',
        value: '<'
      }
    });

})(window.angular);