If I understand, you are looking at backwards. It's the Pandora site that links your account to a Roku. I go to the Amazon Site to register it on the Roku, I go to HBO GO online to register it on the Roku. The Roku gives you a code that you input online, at the site of the company. I could be wrong I don't remember anything that needs to be registered done on Roku itself other than getting a code generated.
Once registered I just click and use it. HBO GO does make you re-register I think every 30 days to be sure you still subscribe with your carrier.
Yesterday I was faced with inputting user id and password on the Roku for Pandora; I walked away to face it again later... Later, as in today, I revisited the Pandora app on Roku to finish getting my account linked up, and the Roku said to go to Pandora.com/roku to get the registration code. I put the code in the Roku and Bob's your uncle! Every day's a learning experience.