Feature require: When any user self-register on my portal, Administrator should have a feature that user may register with "@mydomain.com"

We don't restrict users to register on the portal, anyone can do it. But if you want to restrict users only from @mydomain.com to self-register, you can configure SSO (Remote authentication or Help Centre SAML) where you can authenticate users from your app/website. Here's a link to help you: https://help.zoho.com/portal/kb/articles/setting-up-saml-single-signon-for-help-center#Disabling_SAML_SSO