Signing in to AllHours(AH) is possible with MS account, if accounts/emails are inserted to AllHours.

User sync does not exist, users need to be manually inserted or imported to AH, and if email is same as one in Azure AD-u (office365), sign in will be possible by pressing 'Sing in with Microsoft', without setting password in AH.

At 'Sign in options' you connect your organization

For System administrators - way to set SSO or how to add AH u AAD:

you add AAD admin user to AH. When he tries to sign in, he will receive request that app (Spica Time and Space 360 Level Zero ) wants to access their AAD

Way app looks like in MS Azure:

it is necessary to select only Users.Read

More simple way would be if AH would have access to AAD App Galery, but we still do not have information AH is implemented here.

In case of signing in with MS it is important to add AH as app that can be accessed threw your AAD.