if( ! function_exists( 'videotube_filter_the_submit_login_form' ) ){
    /**
     *
     * Filter the post submit login form
     *
     * @param html $form
     *
     */
    function videotube_filter_the_submit_login_form( $form ) {
        /**
         *
         * Use the 3rd login/registration form.
         * And enter their shortcode below
         *
         */
        $form = '[the custom login form shortcode]';

        return $form;

    }
    add_filter( 'submit_form_login_shortcode' , 'videotube_filter_the_submit_login_form', 10, 1 );
}