Create a new profile
If you are adding a new mailbox or you have migrated from IMAP or Exchange to Microsoft 365, you’ll need to add a new profile before adding the new mailbox to Outlook. Here’s how:
Adding your Microsoft 365 (Exchange Online) to Outlook
Please Note: Outlook 2016,2019 and Office 365 are now only supported, if you have an earlier version you will need to upgrade. If you need to upgrade, please get in touch to upgrade your Microsoft 365 products.
https://learn.microsoft.com/en-us/microsoft-365/admin/setup/setup-outlook?view=o365-worldwide