/**
 * Fires on a successful creation of a sideloaded avatar in the store.
 *
 * @param WP_Comment $comment 
 * @param string     $avatar_url 
 */
function my_sideload_webmention_avatar_callback( WP_Comment $comment, string $avatar_url ) {
    // Your code here.
}
add_action( 'sideload_webmention_avatar', 'my_sideload_webmention_avatar_callback', 10, 2 );

do_action( 'sideload_webmention_avatar', $comment, $avatar_url )