Microsoft 365 Subscription - Where to get Windows 10 product key for offline installation?

Question

I am an admin for a non-profit account. We have licenses for Microsoft 365, which includes Windows 10/11. I am trying to install Windows 10 for a user on new hardware, but am stymied.

• I successfully installed Windows using the media creation tool, but it defaulted to Windows Home, possibly because of an embedded license, and I need to upgrade it to Pro.
• I tried logging in from the newly installed device, with the registered user's Microsoft account, but it complains "That Microsoft Account doesn't exist". In the admin portal I can clearly see that the account exists. When I tried using that account at install time it said "You can't use a work or school address here".
• I logged into the admin portal, as the organization admin, but cannot find anywhere to get a product key. Under the Licenses tab I see "Microsoft 365 Business Premium" but nothing for Windows 10. Selecting that row brings up a list of users who have that license, and selecting the assigned user just brings up a dialog for enabling/disabling apps for that user.

What do I need to do in order to get a product key for Windows 10 Pro?