What is Reddit's opinion of Windscribe VPN - Watch Anything, Privately?
From 3.5 billion Reddit comments

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By popularity on Reddit, this product is:

#2 in Security

Very official already, no need to sideload.

Edit: Also direct linked from the Windscribe website:
https://windscribe.com/download

I don't want to sound like a grouch, but some people don't search themselves before asking others to help them. :(

Amazon app store has windscribe vpn in it. You should be able to install windscribe from amazon app store and set it up.

See the above links.

Yep! Our app is now on the Amazon Appstore so search Windscribe there and you'll be able to download it.

Or you can go here: https://www.amazon.com/Windscribe-VPN/dp/B076ZYV6HQ and simply send it to your Firestick from the browser.