Hi @hip, you can have form data pointing to another form script, as you have done. This is one of the purposes of the Action and Method properties in the form, to allow you to do that. The success and failure messages that show in a normal form submission without redirecting, is Webflow specific code. So those success and failure messages will not work on an external site unless:
A. You are using Webflow to save the form submissions and keep your site active in the dashboard. This will allow you to use those success and failure messages.
B. You create your own ajax jquery code to execute after a form submission to the external mail script handler (in this case the formval-v12.js script) to trigger and show those those success or error states.
Since Webflow does not know the status of whether a form is submitted properly or not to your server, we cannot trigger those messages. This is why we also have the redirect after form submission, to at least allow you to show some message to your visitors
I hope this helps. Cheers, Dave