You can find Norton 360 VPN at the Playstore for easy download or you could search for the apk version of the app and sideload that onto your Formuler box
Sideloading apk/app files is done just a bit different depending on what Formuler model you have.
On the Z8 for instance you'd need to put that apk file into a folder and name that folder 'apk'
You have to do this or the Z8 will not see the file on the storage device.
The other Formuler models do not need that folder.
The 3rd party software (app) that you refer to would be something like an AppInstaller or File Browser or Filelinked.
The Formuler models do have this type of software/app onboard.
I was looking for this 360 VPN but could not find it.
I did see some comments that the 360 does not work on the firestick OS
Regardless thou,I could not find the 360 VPN version in apk form
I did find the Norton Secure VPN APK for Android
Version --> Secure VPN-3.4.6.11962.c8e990f