Step 1: Go to URLs under your Profile and click the Update button to edit. Only account admins can view and change these settings.
Step 2: Create your new URL and click Save Settings. Saving your changes will update the URLs for all forms already created within your account.
Step 3: View the final URL of your form in the Share tab of the form builder. You can create new subdomains at any time using this process.
For more information, please read this help article.