To install unknown apps on your FireStick, you'll need to enable the "Apps from Unknown Sources" option in the Developer Options menu. Here's how you can do it:
1. From the home screen of your FireStick, go to "Settings."
2. Scroll to the right and select "My Fire TV" (or "Device" if you're using an older version).
3. Select "Developer Options."
4. Turn on the "Apps from Unknown Sources" option.
5. A warning message will appear, click on "Turn On."
6. Go back to the home screen, search for the "Downloader" app, and install it.
7. Open the "Downloader" app, enter the URL of the APK file you want to install, and click "Download."
8. Once the APK file is downloaded, click "Install" to install the app.
That's it! You can now install any third-party app on your FireStick using this method.