@d_h01------
You will not be able to access a subscription made directly with Showtime via the Showtime access in The Roku Channel, if that's what you are trying to do.
Install Showtime's own channel app to your Roku (via Streaming channels option on left side of Roku home screen).
Start the Showtime app.
I don't have Showtime so I can't give specifics, but somewhere in there is a way to Login/Signin to an existing Showtime account. If they don't plaster it right on the initial screen, look in the various menu options available, such as Settings, Account, Options, icon that looks like a gear, etc.
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