I just found a silly little workaround for a small problem. Perhaps too small for this blog, but someone will probably find this useful one day.
I am trying to build a link to a page on an internal server. It's not exactly formatted like a standard URL. Let's say it looks like, https://internal/site. I can type this into my browser and it works just fine. But when I try to make a link with a Text or Button Widget, I get an error and Experience Builder won't allow me to click OK.
I assume Experience Builder is using regex to validate the address and is rejecting it because it lacks a high level domain, like .com or .org. (If that doesn't make sense to you, don't worry about it.)