Automatically add a tag when a contact uses a sign up form

Hello,

How do I automatically assign a tag to a contact as soon they fill out a sign-up form? One of the main purposes of using tags is to categorize contacts based on where they're coming from and which sign-up forms they're using. Is this a feature that is available? I see that I can assign contacts to a list when they use a sign-up form but it seems that adding a tag should be a basic feature as well.

Thanks!

Jeremy