https://localhost:44301/Tenant/SignUp is the URI of a resource invoked upon "administrator" sign-in - to a cloud-identity empowered webapp.
The blurb on this page is very confusing, suggesting that the admin of the nth new IDP (now introducing his/her user group to this webapp) should authenticate to Azure portal.
This doesn't make much sense (to me). Surely, the administrator is authenticating to his/her OfficeIDP or AzureAD tenant (or its projected verified domains).
While its true that a "native-Azure AD" is closely associated with an Azure portal, lets recall that in the office365 case signup case the admin may not have such a native-Azure AD (associated with an Azure subscription).
Its FAR More intuitive (initially) for non-technical businesses adopting this stuff to be using an office365 subscription rather than an Azure subscription, to control which webapps can leverages ones users' identities.
Idon't see any need to hide the fact that there is an Azure AD co-resident with each Office365 subscription.