Just implemented an initial version of this. You can find it in the Settings for a Form element:
If the “Redirect URL” is specified, then the user will be redirected to that URL after a successful form submission. If that URL is not specified, Webflow will default to showing the ‘Success’ state message.
If you want to link to another page on your site, you would enter something like /about-page. If you would like to redirect to an external page, enter a full URL like http://www.google.com