Hi there,
I have got the same problem: Some pages restrict the special characters.
I was just about screenshoting LastPass-AddOn as they have had a simple textbox there with the special characters (so edits would be easily possible); but it seems that LastPass has deprecated that feature.
So my suggestion for the UI is just having asimple textbox after the checkbox in which the special characters are listet (and could be changed when needed).
Best regards,
Patrick