Redirect Pending Review Status Preview page url to custom url

    function mmk_pending_review_redirect(){
        $user = wp_get_current_user();
        if(get_post_status ( get_the_id() ) != 'publish')
            if (is_user_logged_in() && !in_array( 'administrator', (array) $user->roles ) && !is_author() ) {
                wp_redirect( "https://example.com/under-approval");
                exit;
            }else{
              wp_redirect( "https://example.com/under-approval") );
              exit;
            }
        }
    }
add_action('template_redirect', 'mmk_pending_review_redirect');

explanation: redirect non-admin users to custom URL if they are not author of the current post where status is not published. (change page id to your custom page)