If you already subscribe to another Microsoft service, such as Exchange Online or SharePoint Online, then you already have an Office 365 account. If you are using Office 365 Enterprise, Government, or Academic, you may be able to simply add Project Online to that account.
How do I know if I have the right kind of Office 365 account?
To check, go to the Purchase Services page in the Office 365 admin center.
To add Project Online to your existing Office 365 account:
Log into your Office 365 account.
On the Office 365 admin center, choose Purchase Services on the left menu.
Choose Add next to the Project Online plan that you prefer, and follow the on-screen instructions.
Note: If you are using a trial version and want to pay for a subscription, choose Buy Now.