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Redirect URL on form submit with custom action not working

I am trying to make use of the custom action post action on Webflow, as demonstrated in this tutorial. I am using a newsletter software called “sendinblue”. The form submit works fine. However, every time I submit the form, instead of staying on the page as I would like to, the submit button redirects me to an empty page from sendinblue. It also doesn’t change into the success message on the webpage – I am not sure how to change that.

I have tried leaving the redirect URL empty or change it into a different page, to no avail.

Here is my site Read-Only: LINK
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