https://www.amazon.com/Windscribe-VPN/dp/B076ZYV6HQ
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.
https://www.amazon.com/Windscribe-VPN/dp/B076ZYV6HQ
https://windscribe.com/guides/firetv
See the above links.
Assuming you have a version 2, go here: https://www.amazon.com/Windscribe-VPN/dp/B076ZYV6HQ
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.