Customizing
Success page
Where visitors land after a submission goes through.
A form posted the ordinary way is a navigation: the browser leaves the page and has to arrive somewhere. Without configuration that somewhere is our own confirmation page, which works but says Formpost rather than your brand.
Send visitors to your own page
contact.html
<input type="hidden" name="redirect" value="https://example.com/thank-you">Any absolute URL works. Set the same thing in the form settings to apply it everywhere without touching markup.
Staying on the page instead
If you submit with JavaScript there is no navigation and no redirect — you get JSON back and render your own confirmation. See the HTML guide for the difference.
Build the thank-you page before you point at it. A redirect to a 404 looks exactly like a failed submission to the person who just wrote to you.