For LaunchBox the Application Path just needs set to "steam://rungameid/253230" (excluding quotes) where that last number is the games app id. If the store page of the game still exists you will find the app id in the url (for example, https://store.steampowered.com/app/237930/Transistor/ the app id you need is the number "237930"). A Steam app ID is ALWAYS numeral, if you see a letter in it you have the wrong thing. Once you add that path to LaunchBox if you play via LaunchBox it will either launch the game in Steam if installed, or bring up the install prompt if it isn't.
Once you have the ID, just press the Add button on the bottom bar (there are other ways as well). This will bring up a blank Add/Edit game form. Just put that launch path as the application path and set the title.
Unfortunately the only way I've been able to find the Steam ID for something no longer with a store page is to install it and make a desktop shortcut for it and then look at the path Steam generates for it. There are some other really technical ways of finding some of the stuff out but it involves looking at some local SQLite DB files and browsing your registry.