Wordfly Automated Welcome Email List Manager Criteria Question

I am in the process of setting up a triggered welcome email for everyone that signs for each of our Wordfly lists. 

I see that our lists live on the constituent's account as Electronic Address Purposes.

So, I can add a list criteria of “Electronic Address Purpose is List Name” to my list manager list, but I am having a hard time figuring out how to populate that list with constituents who just signed up for the email club.

Is the best solution to create a suppression list of people who already have that purpose on their account, then create another dynamic list of constituents with that purpose and then rely on Wordfly to only send the email once to that account? Is there a different way of automating this?

If anyone has implemented something similar I would love to know your list manager criteria to accomplish this. 

Thanks in advance!

Greg