That is something that you normally would not have to do.
Go to https://secure.hulu.com/account and log in.
Scroll down to Manage Devices and remove any that you no longer use.
Then, on your Roku, remove the Hulu app, restart the Roku, then reinstall the Hulu app. All three steps in that order. When you launch and log in, see if the login is remembered.
Note that you may have to log in to other Roku devices (if you have any) after you do this.
Roku Community Streaming Expert
Note: I am not a Roku employee.
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