Restrict your form to only accept submissions from domains that you specify. For example, if "example.com/contact" is the URL where your form is displayed, you can configure your form to only accept submissions that originate from the "example.com" domain . Any submissions from unauthorized domains will be marked as spam.
Referer request header is used to identify the domain where the form was submitted.
Add one or more domains where your form can be submitted. Subdomains of any authorized domains are also considered authorized. For example, by adding "example.com", this also covers "www.example.com" and "subdomain.example.com".