You can’t
An intermediate can issue certs for any domain so unless you are committed to being certified as being compliant to CA/B rules which will require a lot of expensive people and hardware, and can pay up big $$$ to an existing root CA to sign your intermediate, forget about it
Many “custom intermediates” are ones they manage fully for you, and only act as a whitelabeling service so the issuer is “Your Name” and not “EvilTrust Certificate Authority”
