Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Generally, you install Professional Plus from a Microsoft account, as you would with any other version. Your organization (work or school) should be able to provide detailed info.
However, if you got Office as part of the Home Use Program (HUP), now called the Workplace Discount Program, see https://www.microsoft.com/en-us/workplace-discount-program/order-history.