Add an Alias to a User Mailbox as an Admin

An alias is an alternate email address for a mailbox. Everything sent to an alias email address is delivered to the primary email account's inbox. You can create multiple aliases for the same email account.

1. Login to Microsoft Admin Portal
2. If you're not already in the admin center, click the admin app
3. Once in the Microsoft 365 admin center, click Users > Active users
4. Find the user you want to add an alias to and click their Email address or Display name
5. Under the section Username / Email Aliases click Edit
6. Under Aliases, enter the email address you want to add and select what domain the email address will use using the drop down. Click add
7. If you want the new alias to be the primary address of the account, click Set as Primary. If not click Save 

Resources: https://docs.microsoft.com/en-us/office365/admin/email/add-another-email-alias-for-a-user?view=o365-worldwide