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.
It looks like we've already followed up with you directly via email, but here is the information provided for visibility on the forum:
Try reformatting the URL request(s) like this:
Contacts Endpoint POST URL:
https://api.cc.email/v3/contacts?client_id=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx&redirect_uri=https://zapier.com/dashboard/auth/oauth/return/xxxxxxxxxxxxxxx&response_type=token&scope=contact_data
Contacts sign_up_form POST URL:
https://api.cc.email/v3/contacts/sign_up_form?client_id=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx&redirect_uri=https://zapier.com/dashboard/auth/oauth/return/xxxxxxxxxxxxxxx&response_type=token&scope=contact_data
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