In general that can't be done. You need an account with the provider and subscribing through Roku doesn't necessarily get you one. There are exceptions - I know Peacock is one of them. Go to the provider's website and see if you can log in using the email address associated with your Roku account. That's the only information the provider may know about you (they most definitely don't know your Roku account password). In the case of Peacock, entering the email address and choosing "forgot password" was enough to get Peacock to send a password reset which allowed establishing an account that used the same email address that was used for the Roku account but any desired password. That email address and password could be used on any device to log into Peacock.
If that's not possible for whatever service you're trying to access then I'd suggest cancelling your Roku subscription and subscribing directly with the provider. Subscribing through a third party often comes with restrictions.
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