By default, on the free plan, PXM Forms works only on the staging domain (the one with .webflow.io). If you want to use PXM Forms on your custom domain, you would need to upgrade to a paid subscription and authorize it.
How to authorize a domain in the Webflow Extension
Open the Webflow Extension / App.
Go to Settings
Open the Domains accordion
Add the domains you want to authorize
How to authorize a domain in the main application
Go to forms.pixelmakers.com
Open the workspace where you want to authorize a domain.
Open workspace settings.
Find the domains tab
Add the domain you want to authorize.