<form>
<div class="container">
    <div class="row">
        <div class="col-md-offset-2 col-md-4">
            <div class="form-group">
                <input type="text" class="form-control small" asp-for="UserName" />
            </div>
            <div class="form-group">
                <input type="text" class="form-control small" asp-for="Password" />
            </div>
            <div class="form-group">
                <a class="btn btn-sm btn-success pull-right" asp-action="Validate" asp-controller="LogIn" onclick="ValidateLogin()">Log In</a>
                <input type="submit" value="LogIn" asp-action="Validate" asp-controller="LogIn" onclick="ValidateLogin(this)" />
            </div>
        </div>
    </div>
</div>

function ValidateLogin(e:Event) {
    var username = (document.getElementById('UserName') as HTMLInputElement).value;
    var password = (document.getElementById('UserName') as HTMLInputElement).value;
    if ((username.length > 0) && (password.length > 0)) {

    }
    else {
        alert('Fields required');
        e.preventDefault();
    }
}