Using the TV's remote, set the "Power-on" input under Settings->System-Power to the Soundbar's input. The TV should power on to the Soundbar. You may also be able to check "1-touch play" under Settings->System->Control other devices on the Soundbar so that pressing any button other than Power on the Soundbar's remote will turn on the TV and have it automatically switch to the Soundbar.
Roku used to make something called a "Smart Soundbar", but now they have the "Streambar". I only have experience with the latter, but in general, power buttons on Roku remotes are strictly for the TV and have no effect on a Roku device.
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