How Do I Set Or Change The Default Domain In Office 365

You must have at least one domain that added to Office 365 before you are available to choose a default domain.

You cannot change the name of your initial .onmicrosoft.com domain.

Fixes

Step 1: Domain Page

Go to Admin Center, go Settings -> Domains page

Step 2: Select The Correct Domain

On the Domains page, select the domain that you want to set as default for the new email address

Step 3: Default

Select set as Default