If you are saying you have an account and subscription directly with Fox Nation already...
Install the Fox Nation app to your Roku and start it up.
On the first page, click SIGN IN and log in with your Fox Nation credentials.
[Edited to add..] Note, however, that Fox in general has a pretty miserable record of screwing up login issues.
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.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X