Log in to your Microsoft account, depending on whether you already have an older version of Office. Use Microsoft support.

The registration process is not exactly intuitive.
But thanks for the tip. This has helped.

https://setup.office.com/

Yes, download the program as described and enter the licence number when the time comes.
But beware, the page is confusing: you actually install office365 and convert it into a permanent version with a licence key. Just make sure you don't buy an additional 365 licence.
Everything else worked for me and is running tiptop