Hi @miguelst 
welcome!
The opening of a new tab part I’m not clear on. But I do understand the showing of a json response, instead of showing the success message.
I’m assuming you’ve added the Webhook URL to your forms “Action” field, yes?
When you add your own URLs to the forms action field you are telling Webflow that you don’t want them to handle form submissions for you.
What that means is the built in functionality that Webflow provides with forms stops working.
The success/error messages are part of the built-in functionality.
On a bright note, you also get to bypass the monthly form submit thresholds Webflow limits you to.
You’ll notice that you did not receive an email notifying you of a form submit and the forms tab, in your project settings, did not capture the form submit.
Again, you’re telling Webflow that you’re advanced enough to handle all of this yourself.
But, back to your json response…
So you’ve submitted a form to a Webhook URL and thats it. Since Webflow is no longer handling the form submit, it’s up to you to handle the response from that POST request.
The JSON blob you’re seeing is simply what your Webhook endpoint is returning.
I do this often, and sometimes I’ll return a JSON blob in the response that triggers some Javascript to flip on either the success or error message depending on what’s returned (this is fairly straightforward).
Another approach is to just return a 302 response with a “Location” header and redirect the user to another page.
Either work.
If you’re interested in learning more here’s a screencast walking through exactly this using Make (Integromat) Webhooks. It’ll explain a lot of the details about what I’ve mentioned:
Hope that helps!