For anyone still having trouble with this, you can read herefor more information and a video tutorial.

There is no official way to do this. You'd have to unplug the power source. You can use your TV as a remote but that requires setting up and a SmartSwitch. The Roku 4, to date, is the only device with some sort of way to power it off. You can read more about it https://www.orduh.com/turn-off-roku/,