To use Google Chromecast with Prime Video.
You need the most recent version of the Prime Video app.
From the Prime Video app, select the Cast icon.
Select the Chromecast device that you wish to use.
Your iOS or Android device must be connected to the same Wi-Fi.
Go to the Google Play Store and download the primevideo.com/mytv Video app.
After you download the app.
log in with your Prime Video account to watch.
Select “Sign in and start watching”.
Enter your Amazon account information.
Choose “Register on the Amazon website” to get a 5–6 character code.
Then sign in to your Amazon account and enter your code.
Then Find to search for the show or movie you'd like to watch.
Tap on the title you'd like to watch and tap Watch Now.
Your show will begin to play.
Then, tap on Exit playing.