/**
 * Hook to add custom content before the Webmention form added to the comment form.
 */
function my_webmention_comment_form_template_before_callback() {
    // Your code here.
}
add_action( 'webmention_comment_form_template_before', 'my_webmention_comment_form_template_before_callback' );

do_action( 'webmention_comment_form_template_before' )