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Form doesn't Post when set to Post

I have a form in my website that I have set the action to be a MailChimp action URL using the instructions in MailChimp form Integration | Webflow University.

I have set the form to POST:
CleanShot 2021-11-18 at 16.48.20

However, the request is not being sent as a post. It’s sent as a VERY mangled GET request:

Seems like something is misconfigured but I’m not sure what is going on. Can anyone point me to what I’ve messed up?