Xbox and PC are the exact same thing when it comes to the digital copy, just click the "Buy Digital" box and you are good to go
Sea of Thieves pre-order with Amazon for Beta
Not sure when they'll give you the code though, which is usually the biggest problem. I couldn't find a "how/when do I receive my beta code".
I've read about a lot of upset people not getting their beta codes and many that said only 2 business days, but then again...this is the internet.
However, I came across this while I was searching, it may be helpful. IGN broke down the many places selling pre-orders for Sea of Thieves! IGN: Preorder guide Sea of Thieves
Hope this helps.