If you just want to download and use Office on your laptop or desktop, you don't need to add your domain. Just follow these steps.
Just trying to set up your MX record or other records with Office 365? Get step-by-step guidance for setting up DNS records in Office 365 at a variety of domain registrars. Find your registrar on the Create DNS for Office 365 page, and choose Help for detailed steps with screenshots. (Some registrars also have a video option that shows the steps.)
Important: If you’re using Office 365 operated by 21Vianet in China, see What is a domain and how do I use it in Office 365?. You can also see the article Create DNS records for Office 365 when you manage your DNS records.