Any time you specify an action in form setting, Webflow’s default handler is suppressed, and it behaves like a regular HTML form. It does not matter what action you specify, even # or a webhook URL should suppress Webflow’s handling scripts.
However that also means that Webflow’s inline success / fail messages are no longer managed. If you want both - Basin, and the inline success message, you can use Sygnal Attributes.
We use Basin on all of our client sites as well.
Here’s a video walkthrough on how to set that up.