@Meljo, I know in the past when my sister signed up through Roku she was able to go on the Peacock website and attempt to log in using the email address attached to her Roku account (available on your Roku device under Settings->System->About) and use the "Forgot password" option to establish credentials (username and password) she could use anywhere. Whether or not this method still works I couldn't say. She wasn't trying to sign in on a Firestick, but I would think it would work.
Keep in mind that your Roku account password is never shared with any provider, so if you're trying to use that password somewhere else you're doing something wrong.
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