According to this article Belgium is not listed in the available countries:

https://support.sonos.com/en/services/amazon-music

However, you may get it to show if your address in Sonos profile (in web) is in a country where it is available (e.g. US).

Note: I had it and then it disappeared from the music services list (along with Apple Music etc.). It is now back on my list. I re-edited my address in web profile and it still wouldn’t show. It could be that it took over 24 hours to propagate that the app now shows it. So be prepared to wait if you don’t see it after 15 minutes if you change your address. 

Note2: I was able to add the service with Android app but not with iPad app. iPad app kept giving ‘Something went wrong.’ error when authorizing. 
 

Merci pour ce conseil. 
je vais attendre 24h car il n’apparaissait toujours au bout de 15’ 

je testerai également depuis android. 
merci 

Holà, ça a fonctionné au bout de 24h 

j’ai maintenant Amazon music via l’application. 
 

encore merci pour le tuyau

That’s great to hear and thanks for confirming that it just seems to take a little longer than before but the services will then become visible. 
 

Today’s update to the app and firmware seems to have made my android app faster and now Home Screen stays instead of grey boxes like yesterday. 

At least in Amazon Music - likely in all - the issue I still see is that about max. 100 items are shown and not all. When searching for Artist and accessing their releases only about 100 are listed. The old pre-May Sonos app shows more as does Amazon Music app. 
 

I hope Sonos would increase the amount of items shown in those lists.