Just like the announcement instructed, you should select the free offer on the front page.
After clicking that, expect an email to be sent the soonest to be added to your account.
Attached is the picture where you should click it on the GOG homepage. Encircled
the button you should be clicking.
Sorry if I oversimplified your problem. I find no way of knowing exactly what you want.
Hope this solves your problem.