The Roku app on a Samsung TV only provides access to The Roku Channel. Anything that you subscribe to through Roku can only be viewed on a Roku device. You cannot use any other device to view content subscribed through Roku.
Some providers might permit logging into their content through their own app using credentials that you get from your Roku subscription. This is not your Roku user name and password, but some sites will allow you to create credentials on their site using your Roku subscription as your provider. I have no idea if Starz might do that, but my personal guess is the answer would be no.
