Pretty sure it has to do with the security store config in Portal Admin:
https://webadaptorhost.domain.com/webadaptorname/portaladmin
From there you'd go to: Security > Config...and if I'm wrong, someone please correct me...
By default, Portal uses a built-in store for both users and groups (named accounts). I feel pretty confident that if we were to change this to use our Active Directory, using my domain account would work.
We have kept it as a built-in user store as we may have outside organizations requesting access to web maps, services, etc... We have configured a SAML login using our Active Directory (Portal Home > Organization > Edit Settings > Security > Enterprise Logins via SAML), so this hybrid approach works well for us, where our County employees can use their domain accounts, and any outside user would be a named account.
If you want to change the security config of Portal, this link will provide you with examples on how to do that.
Keep in mind, if you are using a domain account, or the like, you'll probably want your IT to setup an account with a password that does NOT expire.