Ok, I took a different approach
This code fires on the AfterInsert event. So when I hit TAB and go to a new record, voila.
FieldName.DefaultValue = """" & [FieldName] & """"
I'm not sure it's best practices, but whatever. Google was surprisingly silent in answering this question.
Not OP, but I was following along and now I'm stuck.
I have the FieldName appearing in a Message Box, but how do I get that value into the new record that hasn't been written yet?