Hi @kayalg12,
Welcome to the Community Forum, we are so happy to have you here!
This is a great question as API’s can be tricky sometimes! You are correct, signup forms only take a profile’s email address. However, you can set up an endpoint to retrieve the profile’s ID from their email address. I recommend using the URL to do so:
https://a.klaviyo.com/api/v2/people/search?api_key=API_KEY_HERE&email=EMAIL_HERE
Additionally, I’d suggest checking out this Community Post for more on retrieving person ID’s using an email address or in case you run into some troubleshooting issues as you install.
For more on Profile ID’s I’d recommend checking out this Help Center Article and Community Posts to grow your understanding about APIs and how they work!
Thanks for being a part of the Klaviyo Community!
-Taylor