When the insert behaviour is used, it creates the new unique ID in a session.
You can then use this session to filter a new recordset to select the new record.
Then use the details from the recordset to populate your UE fields.
Essentially you are on the right path, but you need to load the information into a recordset.
There are other ways around it of course, in that you could store the POST value of the email field in a session and use that. So your POST behaviour would trigger:
i) Store the POST value of the email field in a session
ii) Insert the record using the WA Insert Behaviour
iii) Use the session that stores the email address as the to address in UE.
It all depends which route you want to go down.
Personally, I would go with the creation of the recordset filtered by the new unique ID created by the insert behaviour.