The only way the TV input can change to the one used by the Roku is if your TV supports CEC. Most, but not all, do, but you might need to enable it on the TV. For the Roku it's labeled One-touch play, and normally it's enabled by default. But I would confirm it's turned on in the Roku settings as well.
If your TV doesn't support CEC, then you have no option but to switch inputs with the TV remote.
Dan
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