Membership2 - Use default registration page

ms-default-registration-page.php

<?php
/*
Plugin Name: Membership2 - Use default registration page
Plugin URI: https://premium.wpmudev.org/
Description: Use the WordPress default registration page
Author: Panos Lyrakis @ WPMUDEV
Author URI: https://premium.wpmudev.org/
License: GPLv2 or later
*/
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

if ( ! class_exists( 'WPMUDEV_MS_WP_Default_Reg' ) ) {

	class WPMUDEV_MS_WP_Default_Reg {

		private static $_instance = null;
		private static $_reg_url = null;

		public static function get_instance() {

			if( is_null( self::$_instance ) ){
				self::$_instance = new WPMUDEV_MS_WP_Default_Reg();
				self::$_reg_url = 'http://site.com/path-to-memberships-list';				
			}
			return self::$_instance;

		}

		private function __construct() {

			add_filter( 'ms_frontend_handle_registration', array( $this, 'cancel_registration_redir' ), 10 );
			add_filter( 'ms_model_pages_get_ms_page_url', array( $this, 'get_ms_page_url' ), 10, 4 );

			add_action( 'register_form', array( $this, 'register_form_fields' ), 10 );
			add_action( 'login_form', array( $this, 'register_form_fields' ), 10 );
			add_action( 'register_post', array( $this, 'redirect_on_registration_error' ), 10, 3 );
			add_filter( 'login_message', array( $this, 'registration_errors' ), 10 );
			
			add_action( 'user_register', array( $this, 'user_register' ), 10 );
			add_action( 'ms_show_prices', array( $this, 'ms_show_prices' ), 10 );
			
			add_action( 'login_url', array( $this, 'login_url' ), 10, 3 );
			add_filter( 'login_redirect', array( $this, 'login_redirect' ), 10, 3 );

		}

		public function cancel_registration_redir(){
			return false;
		}


		public function get_ms_page_url( $url, $page_type, $ssl, $site_id ){

			if( $url == self::$_reg_url ){
		        $url = wp_registration_url();
		    }

		    return $url;

		}


		public function register_form_fields(){

			if( isset( $_REQUEST['_ms_action'] ) && $_REQUEST['_ms_action'] == 'membership_signup' &&
        		isset( $_REQUEST['membership_id'] ) && is_numeric( $_REQUEST['membership_id'] ) ){

        		foreach( $_REQUEST as $field_key => $field_val ){
        			if( $field_key == 'reg_errors' ){
        				continue;
        			}
            	?>
           			<input type="hidden" name="<?php echo $field_key; ?>" value="<?php echo sanitize_text_field( $field_val ); ?>" />
            	<?php
        		}
    		}
		}


		public function redirect_on_registration_error( $sanitized_user_login, $user_email, $errors ){

			if( ! isset( $_REQUEST['_ms_action'] ) || $_REQUEST['_ms_action'] != 'membership_signup' ){
				return;
			}
			
			if ( $errors->get_error_code() ){

				$_registration_errors = urlencode( json_encode( $errors->errors ) );

				$redirect_url = wp_registration_url();
				$url_query_string = '';
				$query_args_list = array( '_wpnonce', '_wp_http_referer', 'membership_id', '_ms_action', 'step', 'submit' );

				foreach( $_POST as $field_key => $field_val ){
					if( ! in_array( $field_key, $query_args_list ) ){
						continue;
					}
    				$url_query_string .= '&' . $field_key . '=' . $field_val;
				}

				$redirect_url .= $url_query_string . '&reg_errors=' . $_registration_errors;
				wp_redirect( $redirect_url );
		        exit;

			}
			
		}


		public function registration_errors( $message ){

			if( ! isset( $_REQUEST[ 'reg_errors' ] ) || $_REQUEST[ 'reg_errors' ] == '' ){
				return;
			}
			
			$error_log = json_decode( stripslashes( urldecode($_REQUEST[ 'reg_errors' ] ) ) );
			$error_message = '';
			foreach( $error_log as $error_key => $error_msg ){
				$error_message .= $error_msg[0] . '</br>';
			}

			$error_message = '<div id="login_error">' . $error_message . '</div>';
			return $error_message . $message;
		}


		public function user_register( $user_id ){

			if( isset( $_REQUEST['_ms_action'] ) && $_REQUEST['_ms_action'] == 'membership_signup' &&
		        isset( $_REQUEST['membership_id'] ) && is_numeric( $_REQUEST['membership_id'] ) ){

		        wp_set_current_user($user_id);
		        wp_set_auth_cookie($user_id);

		        $membership_id = (int)$_REQUEST['membership_id'];
		        $url = self::$_reg_url . '?step=payment_table&membership_id=' . $membership_id;
		        
		        wp_redirect( $url );
		        exit;
		    }
		}


		public function login_url( $login_url, $redirect, $force_reauth ){

			$url_query_string = '';
			$query_args_list = array( '_wpnonce', '_wp_http_referer', 'membership_id', '_ms_action', 'step', 'submit' );

			foreach( $_REQUEST as $field_key => $field_val ){
				if( ! in_array( $field_key, $query_args_list ) ){
					continue;
				}
				$url_query_string .= '&' . $field_key . '=' . $field_val;
			}

			return $login_url . '?' . $url_query_string;
		}


		public function login_redirect( $redirect_to, $request, $user ){

			if( isset( $_REQUEST['_ms_action'] ) && $_REQUEST['_ms_action'] == 'membership_signup' &&
		        isset( $_REQUEST['membership_id'] ) && is_numeric( $_REQUEST['membership_id'] ) ){

		        $membership_id = (int)$_REQUEST['membership_id'];
		        $redirect_to = self::$_reg_url . '?step=payment_table&membership_id=' . $membership_id;

		    }

		    return $redirect_to;
		}

		public function ms_show_prices(){
			?>
		    <script type="text/javascript">
		    (function($){
		        $(document).ready(function(){

		            $('.ms-membership-form').on('submit', function () {

		                if( $( this ).data( 'has-been-reset' ) == 'true' ){
		                    return;
		                }

		                $(this).data( 'has-been-reset', 'true' );
		                /*Changing form method to GET so the registration page doesn't check for post var and throw error message*/
		                $(this).prop('method','get');
		                /*Changing action name so it doesn't replace the ?action=register*/
		                $(this).find('#action').attr( 'name', '_ms_action' );

		                $("<input>").attr({
		                    'type':'text',
		                    'name':'action'
		                }).val('register').appendTo($(this));

		            });
		               
		        });
		    })(jQuery);
		    </script>
		    <?php
		}

	}

	add_action( 'plugins_loaded', function(){
		$GLOBALS['WPMUDEV_MS_WP_Default_Reg'] = WPMUDEV_MS_WP_Default_Reg::get_instance();
	}, 10 );
}

Please remember to replace line
self::$_reg_url = 'http://site.com/path-to-memberships-list';
with your memberships list path