As you might have known, BattleForge used to be available on Steam both as a demo and a full game. Even now that the game has been removed from Steam, you can still download the demo (the original version that comes with the retail DVD). This is a short guide on how to play SkylordsReborn on Steam using that demo.
1. Download and extract SkylordsReborn to your PC, say D:\BattleForge.
2. Run Steam in background. Open up your browser (Firefox, Chrome, etc.) and type this into the address bar: steam://install/24770
A popup menu will ask if you want to open that URL with a specific application, select Steam Client Bootstrapper.
Steam should start downloading BattleForge demo (~2.2GB).
3. Run the demo from Steam once to start the installation process. Once the launcher shows up, close it.
4. Copy everything inside the folder from step (1) to the demo installation location, replacing any duplicated files.
e.g. from D:\BattleForge
to D:\Program Files (x86)\Steam\steamapps\common\Battleforge
5. Delete any extra files that are in the demo installation folder but not in the SkylordsReborn folder (from step 1) except these 3 files:
steam_appid.txt
settings.ini
Bootstrapper.exe
6. Rename Bootstrapper.exe to Bootstrapper.exe.org (think of it as a way of backing up).
7. Open a command prompt (as admin if you installed the demo in drive C:\), type this and press Enter:
mklink "D:\Program Files (x86)\Steam\steamapps\common\Battleforge\Bootstrapper.exe" SkylordsRebornUpdater.exe
This should create a new symbolic link called Bootstrapper.exe that will run SkylordsRebornUpdater.exe.
8. Done. You can now launch SkylordsRebornUpdater by launching BattleForge demo from Steam (just click Play). Steam will try to run Bootstrapper.exe which will just run SkylordsRebornUpdater.exe.
Steam chat should display your profile as "Playing BattleForge Demo" (not playing non-Steam game) and will continue to do so until you exit BattleForge (exact behavior as any other Steam games).
Your playtime will also be recorded in your Steam profile.