Can you make it monospace by default, so that this can be used as a code snippet bin?

Sure! textarea.my support custom style attr: https://textarea.my/#Ky4tSlVUyCotLlEoLUhJLElVKC6pzElVSCwpKWJ...

How do you share after that? I can open devtools and change the attribute but the URL doesn't update after that.

Update the text a bit to trigger save event.

This is a code editor with the same url idea:

https://flems.io/