Firstly install Amazon app if you haven’t already. Do login with you regular amazon account.
Then visit this link via Chrome.
When the the link opens in chrome then click on the “Get Amazon Underground” yellow color button from chrome, it will open amazon app and ask for GPS On/Off, simple deny and the games shud be visible now.
There is no Amazon Underground or games directly visible by opening Amazon app. You have to visit the webpage link and click from there to get into games area in Amazon app.