Here is the link:

https://www.amazon.com/music/unlimited/hd

Please note I'm not affiliated with Amazon. I also don't plan on signing up at present. I'm a happy Tidal and Qobuz via Roon streamer. : )
Usually I'm not too thrilled about Amazon taking over markets, brick and mortar, etc.  And I'm no expert on the economics of this.  That disclaimer aside, it does seem like it will put market pressure on all to stream higher resolution and that is a good thing for the hobby. 

Off to check out the terms. 
I've signed up now trying to figure out how to get it to work on my system. I'm using JRiver on my laptop and bubbleupnp on Android tablet. I'm going load the Amazon music app on to the laptop.  If anyone is successful please post instructions.