Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
As you mentioned, there are several different ways that a tag could be added using one or more additional calls, but at this time there is not a way to include the Tag value when making POST calls to the /contacts/sign_up_form endpoint. While no error is returned if the value is included within the request body, the endpoint ignores that data and does not add the tag.
An easy alternative would be to use additional lists, one as the desired mailing list, and one as the placeholder for the tag, and then have the call add the sign-up to both lists.
I was unable to find any information specifying why the Tag value was not included when creating the request body schema for the Sign-up Form endpoint, but have passed along your request to our developers for consideration.
Your feedback and experience with this request is essential to improving our product, so thank you for reaching out to us regarding this matter.
Please have a look and let us know if you have any other questions!
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