Assuming you're talking about a RichTextLabel, the only way I see it is have the
on_meta_clicked method connected to that signal, replace the bbcode between that [url] tag, with the same text, but wrapped with the extra bbcode tag you need. E.g. you could turn it underlined or change the font colour. Unfortunately the node does not offer a native way to change font background, but you can develop a custom effect to do that youself.