I just got the free year of Apple TV . How can I watch it on Roku? Is there a password?

If your Roku model is supported, then you may download the Apple TV app from the Roku Channel Store. Next, open the Apple TV app, find the in-app settings (sprockets wheel icon), and sign in with the Apple ID account that has the subscription.

Watch in the Apple TV app on Roku - Apple Support

Apple TV app-supported devices - Apple

I always just use the apple TV app on phone & screen mirror to TV screen directly, must be a smart TV though...