wpmem_restricted_msg

/**
 * @param string $str    The post restricted message with HTML.
 * @param string $msg    The raw message string.
 * @param string $before The 'before' HTML wrapper.
 * @param string $after  The 'after' HTML wrapper.
 */
add_filter( 'wpmem_restricted_msg', function( $str, $msg, $before, $after ) {

    /*
     * This example replaces the default <p> tag in the message
     * string with one containing a custom class.
     */
    
    $str = str_replace( '<p>', '<p class="my-custom-class">', $str );

    return $str;
});