Apple TV 90 day trial

Do as Best Buy says on how to redeem. Apple Free Apple TV+ for 3 months (new subscribers only) - Best Buy

• First get a proper Apple ID account, if you do not have one yet. It requires an Apple ID with payment method on file.
• I think you should get an email with redeem instructions and a code from Best Buy.
• Download the Apple TV app on Roku.
• Sign in to the Apple TV app with the Apple ID account.
• Enjoy Apple TV+ (one of the Channels in the Apple TV app).

Apple TV app user guide for third party streaming devices - Apple Support

Apple TV+ in the Apple TV app - Apple Support

Roku is not an Apple product, best you ask the app developer on their website/forum for support.

You have to ask Roku or Best Buy