It might vary by brand. Who makes your Roku TV? I believe my Sharp has a power button sort of hidden in the back, but I honestly don't remember. Since the TVs support IR remotes, you should be able to buy a generic IR remote that will work with it. Just check the packaging to see if Roku is listed.
Nvidia Shield, Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.