This schedules a routine to run at startup. Fill in the routine with the code you want, and it will run at DEFAULT_PRELOAD_PRIORITY which is the number 69. 

  
PRELOAD( MyCustomInit )
{
    // do something here (do anything here,
    // without limitations that are imposed by DllMain/LibMain.
}

If you wanted a routine which was guaranteed to run before MyCustomInit you might use PRIORITY_PRELOAD whcih allows you to specify a priority.

  
PRIORITY_PRELOAD( MyOtherInit, DEFAULT_PRELOAD_PRIORITY-10 )
{
   // this will run before other things.
}

Priorities are listed in deadstart.h and exit_priorities.h. The priorities are treated backwards, so low number startup priorities go first, and higher number shutdown priorities go first.