Go to ‘Setup > Domains’ (if this is a new Office 365 service) or ‘Settings > Domains’ on the left-hand menu
Select Add Domain
Enter the domain you wish to add and press Next
You will be prompted to verify ownership of the domain. Depending on who your domain is registered/hosted with, you may be prompted with different options:
If your domain is with GoDaddy or 1&1, you will have the option to sign in with them, which will redirect you to their login page for authentication.
If your domain has a contact email address published, you can have a code send to that address, which you can use to verify the domain.
If the first two options are not available, you will be asked to add a record with your domain host. NOTE: You should only add the TXT record for verification at this stage. Instructions for adding the TXT record can be found here.