Add unique ID number to end of permalink

by

I think you use the function wp_insert_post() from the form to create the post; the function return the ID and you can use it. 

    if ( defined( 'WP_DEBUG' ) && WP_DEBUG )
        $wp_error = TRUE;
    // insert post
    $post_id = wp_insert_post( $new_postarr, $wp_error );