You can have multiple custom domain names for each tenant, so adding another one is simple, you just add it and confirm ownership.
Making it the default is a little more complicated. You can change it to default in the UI, but you may then have to go through existing users and change their upn suffix if you want to use that.
Possible duplicate "What Does the “Use this account everywhere on your device” Dialog Do?"
To answer your question, since you unchecked the checkbox and clicked "This app only", you effectively only signed in to that OneNote app with your Microsoft account and did not join the organization's Azure domain. Refer to the above link for more detailed ...
To call the refreshes API the service principal must have admin permissions in SSAS. This is not accomplished through the Azure portal but through SQL Server Management Studio as described here.
Once the service principal is created, its application ID can be
assigned permissions in the Azure Analysis Services server or model
roles using the ...
Create an empty OU in your on premises AD.
Reconfigure Azure AD Connect to synchronize only this OU. This will delete the Office 365 users that were synched incorrectly.
Connect to Office 365 with Powershell and remove the users from the Office 365 recycle bin:
(Remove-MsolUser -UserPrincipalName First.Lastname0815@companycom.onmicrosoft.com -...