Hello @ChrisC342 ,
For the coded inline or pop-up forms, the Thank you page isn't editable at this time. You can edit the Thank You page of a landing page form, which includes its wording as well as redirect link or attachment download. If you're building your own sign up form and trying to connect it with our API, you may want to reach out to our Web Services / Developer support on their Community board or main site.
For voicing of use cases and voting on different coded form customizations, I'd recommend checking out this Feedback thread.
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