Hello @GiuliaP89 ,
At this time there isn't a way for a sign-up form to automatically attach a tag, it'd require some degree of manual work. You can go into the list they're assigned to, select-all the list, and add the tag. If you use a sign up form system detached from your Constant Contact account, and have to import the contacts manually via a file upload, then you can include the tag as part of that file upload.
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