First, when you say "How do I add Prime Video to my Roku channel", do you have a Roku device or are you viewing "The Roku Channel" on a non-Roku device? If so, you can't add anything to The Roku Channel; you need an actual device.
If you have a Roku device, are you in the U.S.? If not, it may not be available via Roku in your area. You should contact Amazon and ask them about it.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution." If you appreciate my answer, maybe give me a Kudo.
Also, What Roku model and model number do you have (Settings > System > About)? Amazon did not code their Prime Video app to work on some older Roku models.
"Prime Video is available for Roku 2 (except for models 2720x and 27020EU), Roku 3 (except for model 4230x), Roku Express, Roku Express+, Roku Streaming Stick (except for models 3400x, 3500x and 3500EU), Roku Streaming Stick+, Roku Premiere, Roku Premiere+ and Roku Ultra as well as Roku TVs."
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking " as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X