[oauea]

And most importantly, don't let yourself get into the habit of copy pasting the value

[underwater]

I wonder if your could print some non visible characters in there to taint the copied value in some detectable way.

[dkersten]

Prompt in words, but expect the value in numbers, eg: "Twenty-five" and the box requires you to type "25"? At least this specific case, it would require you to type it.

[weaksauce]

yeah, that would possibly stop the copy and paste problem. to make it robust they would need to use a string of a few non-visible characters but that would fail if the browser's clipboard system doesn't copy them over for some kind of privacy initiative. might be another way it fails that I can't think of right now.