set input field to content of notes field?

set input field to content of notes field?

I've created a user agreement form, which has a note field that has the user agreement text.  I would like a hidden field that gets assigned the contents of the note field that has the user agreement text upon submit (or load).

If the agreement language changes, I need to have a copy of the exact text that was in place when each person "signed" the agreement.

When I try to do this on validate, I get this error when trying to save:
Field plain does not exist in the form User_Agreement

When I try to do it on load or success, I do not get an error, but when I test it out, my field it not getting updated with the note text.

Any ideas on how I can get a copy of that text?

Thanks in advance!