Prompt user. Maybe they don't want it, but do it by default.
https://developer.valvesoftware.com/wiki/Steam_browser_protocol
If you want, you could add SPV3 to Steam via steam://AddNonSteamGame during the install procedure
Steam browser protocol
I'm wondering if there's a way to auto-set the three custom images as well
Contact Arecadian Fox and Jazz for the SPV3 Steam Library images.