If the form is on a COS page, then i think it is easier than you think (you won’t even have to do anything api related)
- Go to edit the form module
- Even if you are going to an internal page, click redirect to external URL
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- redirect to your desired page and attach whatever contact property you desire via query parameters.
http://redirect-address.com/the-page?token={{contact.your_internal_property_name}}
After you click save, you can see your link when you hover over test link
