Some more questions, @Thorsmom66…
Is this a mandatory field? If not, skip it.
Is it a text field? If yes, you can write “market pay”.
Is it a range?
If none of the above (meaning, it’s a mandatory numeric field), make an educated guess. It really doesn’t matter for internships - most companies have a predetermined number and that’s what they’ll pay regardless of what you say in the application. It’s likely a holdover from their common application form.