Cakestax To get the default directory on C: you can use %ProgramFiles(x86)%\Steam\steamapps\common
as your prefix.
But why do you need that? Keep in mind that people often have more than one location where they install their games. For example I have a Steam apps folder on drive C: and E:.
If you really need to know where someone installed a game you should ask them and let them pick a directory. Although of course it cannot hurt to check some default locations and try to find it yourself first but in the end you should ask.