After you have purchased Office you will need to activate and install the Purchased product. For that Microsoft has set up office.com/setup.
One of the main use of the office.com/setup website is to let Office users Sign In and Activate their product key. After you enter the product key, your Purchased product will be added to your My Account page for users to download and setup on their Mac or PC.
Other similar URL’s are setup.office.com, office.com/setupkey, Microsfot365.com/setup.
You can visit office.com/setup from any browser. You will need a Microsoft Account and an active Office subscription.