Thank you for reaching out to Constant Contact API Developer Support and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
We're still in the process of rolling out new endpoints for Tags support in the V3 API.
It looks like a bulk activity endpoint to remove Tags has been published, but as of just yet, there is no bulk endpoint for adding Tags: https://developer.constantcontact.com/api_reference/index.html#!/Bulk_Activities/postTagRemoveContact
There are also Tag capabilities in the body schema for creating or updating an individual contact, but none for the sign_up_form endpoint currently: https://developer.constantcontact.com/api_guide/tags_overview.html
We look forward to making new endpoints available for Tags. You can keep up to date on the latest available V3 endpoints here: https://developer.constantcontact.com/api_guide/release_notes.html
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