function cognito_login(username, password) {

    var authenticationData = {
        Username : username,
        Password : password
    };
    var authenticationDetails = new AWSCognito.CognitoIdentityServiceProvider.AuthenticationDetails(authenticationData);
    var userData = {
        Username : username,
        Pool : AWSC_userPool
    };

    var cognitoUser = new AWSCognito.CognitoIdentityServiceProvider.CognitoUser(userData);
    cognitoUser.authenticateUser(authenticationDetails, {
        onSuccess: function (result) {
             return true;
        },

        onFailure: function(err) {
            console.log(err);
            return false;
        }
   });
}

from flask_wtf import FlaskForm
from wtforms.fields import TextField, BooleanField, PasswordField
from wtforms.validators import DataRequired

class LoginForm(FlaskForm):
    username = TextField('username', validators=[DataRequired()])
    password = PasswordField('password', validators=[DataRequired()])

    def validate(self):
        if not FlaskForm.validate(self):
            return False

        ... javascript here ...