Second email designated by first participant

Hello, I'm trying to create a Web Form where the first participant designates the email of a second participant. In other words, I have a form I need two signatures on, but I don't know either party's email in advance. The first party will always know the second party.

 As far as I can tell, the most recent help topic on Web Forms, here, indicates this should be possible. I have an Adobe Acrobat Pro plan subscription.

However, when I "add participant" on my web form, it requires an email for the second participant.

I assume this is related to the recent end of Adobe Sign as a separate product... but can I still get this functionality somehow?