"InstallScript"
{
   "Run Process"
   {
      "dx8dll"
      {
         "HasRunKey"      "HKEY_LOCAL_MACHINE\\SOFTWARE\\Valve\\Steam\\Apps\\_YOUR_APP_ID_"
         "process 1"      "%INSTALLDIR%\\dx8dll.bat"
         "command 1"      "/silent"
         "NoCleanUp"      "1"
      }
   }
}