To download 3rd party apps on the firestick we first have to download "Downloader". There are a few steps that we need to take before we can complete our tasks.
Enable the Firestick to allow apps from unknown sources:
Open the Firestick
Go to Settings > System > Developer Options.
Select “Apps from Unknown Sources”
Turn the option to “ON”.
Now exit settings
Now add the Downloader App:
Go to the home and select Search
Search for "Downloader"
Select Download or Get
Wait for the download is complete notification.
Once downloaded. Go to Apps on the homescreen.
Launch the Downloader app