Domain restrictions are only available on the Professional and Business plans.

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Configuring your forms domain restrictions

Go to your form settings and scroll down to the Security section. Here you can add the domains and subdomains you want to restrict your form to.
In order to use Formbox’s domain restrictions you must ensure your site’s Referrer-Policy sends your site’s origin information to Formbox.By default, sites usually have the Referrer-Policy set to strict-origin-when-cross-origin which is compatible with Formbox. Learn more about referrer policy here.