/**
 * Filter Comment Data for Webmentions.
 * 
 * All verification functions and content generation functions are added to the comment data.
 *
 * @param array $commentdata 
 * @return array|null|WP_Error $commentdata The Filtered Comment Array or a WP_Error object.
 */
function my_webmention_comment_data_callback( array $commentdata ) {
    // Your code here.
    return $commentdata;
}
add_filter( 'webmention_comment_data', 'my_webmention_comment_data_callback' );

apply_filters( 'webmention_comment_data', $commentdata )