/**
 * Callback function for the 'webmention_before_reaction_list' action.
 */
function my_webmention_before_reaction_list_callback() {
    // Your code here.
}
add_action( 'webmention_before_reaction_list', 'my_webmention_before_reaction_list_callback' );

do_action( 'webmention_before_reaction_list' )