Unknown sources on an Android TV: where the setting is
Out of the box, an Android TV or Google TV installs apps from the Play Store and nowhere else. One switch changes that, per app: a file manager, a downloader or a browser is allowed to install what it fetches, and nothing else on the set changes. Watchcric itself has no TV app and never asks for this; the page is here for anyone putting some other app on a television.
Most sets need only the first. Try the second if the first is not there, and the third only if the second fails as well.
1From the warning on screen
When the TV says the app is not allowed to install unknown apps, pick Settings on that warning rather than closing it. A list of apps appears; switch on the one you are installing from, press Back, and the install carries on.
If there was no warning, or Settings was not offered on it
2Through the Settings menu
The same list can be opened by hand; sets file it under different headings. Try each in turn:
Settings → Apps → Security & Restrictions → Unknown sources Settings → Device Preferences → Security & Restrictions → Unknown sources Settings → Apps → Special app access → Install unknown appsWhere Settings has a search box, typing unknown finds it fastest.
If the setting is in none of those places
3Turn on developer options first
A handful of devices keep the switch hidden until developer options are on, among them the Google TV Streamer, the Chromecast with Google TV and some TV brands.
Open About in Settings (under System on Google TV, under Device Preferences on older Android TV). Go to the bottom line, Android TV OS build (Build on older sets), and press OK on it seven times. The set counts down and ends with "You are now a developer".
Go back to the first or second way: the switch is there now.
Is it safe?
For the one app you switch on, yes
Installing from outside the Play Store is built into Android; every other app store on the platform depends on it. The permission is per app, so nothing else gains the right to install, and Play Protect goes on scanning as before. Grant it to apps you trust, and switch it off again when you are done if you prefer.