To modify and create a more user-friendly URL for your Form, go to My Account > URLs and create or edit a URL Prefix. To see more on adding or editing these URLs, please check-out our User-Friendly URLs help guide.
At this time we do not support customizing the Form URL addresses through the Formstack app, however, you can customize the URL by Embedding the Form onto your own website in order to use a fully custom domain.
Note: Some domains offer "stealth forwarding" services that allow you to mask a URL with a different one to fake a custom domain. This method usually loads the original URL within an iframe. This will cause any Paypal WPS redirects set up not to function.