Something like e.g. patreon link or link to my website. Do I just use raw HTML or what?
You can’t put one into the game itself. But if you’re using the Inform 7 template, you can edit the sidebar.
Worst case scenario - just spell out the URL in text and hope the interpreter allows copy paste, or use a website that’s not difficult to type in manually.
I have been thinking a Glk extension for a simple hyperlink function would probably be an easy thing to introduce.
It’s not a bad idea for interpreters to auto-detect URLs in the game output and linkify them, the way text apps generally do nowadays. (I mean note-taking apps and so on.)
I implemented this feature for Quixe/Lectrote at one point, but it’s not turned on by default. Maybe it’s time for that.