Hi, Jake B.
I have searched on documentation, but found nothing. I think that, the default behavior of drag and drop is not to create a input widget for the identifier, whether it is text or not. It is not common to allow the user to create an identifier. Have you tried to create an input widget by yourself and then to set the variable to the identifier?
Nonetheless, I think you should consider to use the default entity identifier and set an unique index to the field you want as identifier. It can save a lot of work now and in the future.
Hope it helps.
Best regards.