It’s just hyperlinks that it’s picky about. (The old forms didn’t allow them either.) But because these new forums automatically try to hyperlink anything that looks like a URL, it’s erroring when you try to submit.
To get around it, you just have to fool the forum into thinking it’s not a URL. The cleanest way is to use the Preformatted Text markdown on the URL. That’s the </> symbol on the toolbar.
For example
www.google.com