It's more of a tool I think, but it's classified as game on Steam. Anyway, if you add it this way it is free.
Couple methods to do it:
Click the green install button on the SteamDB page
AddFreeLicense(643141) for browser
ArchiSteamFarm: !addlicense asf s/643141
Steam store page for reference (not free): https://store.steampowered.com/app/1543590/Wildshape__Map_Editor__VTT/