The source input is usually a button on your remote ... so you would switch that to the HDMI-ARC port number, (Example HDMI-3 or whatever one you have plugged your Beam into) before then giving the instruction 'Alexa, turn off the TV' ... although the source is also accessible via settings ... I would try that.
If that fails and you do look at the Harmony Hub.. it will do the following;
Turn TV on/off
Adjust volume up/down/mute
Pause/Fast Forward/Rewind TV if your TV supports that.
Change TV channels .. upto 50 favourite channels can be chosen
Change input sources
Launch Apps built into the TV (Netflix etc.)
Control other devices attached to the TV (Apple TV, PlayStation and many others)
Adjust Hue lights in the TV viewing room.
All above can be done via Alexa by adding the Harmony Skill to the Alexa App... all the TV setup is done on mobile devices via a free downloadable Harmony App... see the two screenshots from my own setup.
The Hub also works with Google Home and with Sonos speakers direct using the Sonos API .