You never enter your Roku account email address and password on a Roku device. I would say to enter whatever email address and password you're using on the Paramount+ website, but it appears you can't log in there either. When an app asks you for an email address it suggests the email address you're using for your Roku account, but you can enter any email address you like.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.