The below steps will guide you through adding an additional mailbox to your Outlook.
From within Outlook, in the upper left corner click File, then Account Settings, and Account Settings again.
Within the Account Settings window, click on New to start the Add Mailbox wizard.
In the wizard, enter the email account you are trying to add, then click Connect.
When prompted, choose the Microsoft 365 account type.
Enter the password, and click Sign In.
Finally, click Done, then Close in the Account Settings window. You will need to restart Outlook for the changes to take effect.