I've tried the following and it works for me.

lfilename='c:\test.bmp'
*!* declare the shellexecute parameters
DECLARE INTEGER ShellExecute ;
IN SHELL32.DLL ;
INTEGER nWinHandle,;
STRING cOperation,;
STRING cFileName,;
STRING cParameters,;
STRING cDirectory,;
INTEGER nShowWindow

*!* execute Printto option for shellexecute
*!* where lfilename = document to be printed
*!*
LOCAL lretval
lretval=ShellExecute(0, "Printto", (lfilename), ["]+SET('PRINTER',3)+["], 0,1)