I'm a bit confused. Your title talks about accessing FYC Prime Video, but in your text you want to install regular Prime Video.
I never heard of FYC Prime Video so I did a web search and found this: https://www.amazon.com/Deluxe-Entertainement-Consider-Amazon/dp/B08D6W7V12
This sounds like this is an app Amazon has released for Android devices running the Amazon Appstore app. There is no mention of availability on Roku.
As for regular prime Video, there is a Roku channel app for this you can install from the Streaming channels option on your Roku TV home page. This requires a paid subscription. This can be obtained as part of an Amazon Prime subscription, or I think I've read you can subscribe to Prime Video only. When the app is installed and loaded click on the "gear" icon in the upper right and Sign In into your Prime account.
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 "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X