We've seen many customers who asked if they can tag all subscribers who signed up from a particular sign up form.
...and the answer is Yes! and the best of all, it's easy to implement.
You have to follow the steps below:
Create a form and save it. E.g. "My-form"
Create a tag. E.g. "My-tag"
Create an automation from scratch with one trigger and one action like below:
-Trigger: subscriber joins form > select "My-form"
-Action: add tag > select tag "My-tag"