Use the TCL Global variable $FME_MacroValues(). Many Global variable are available in the Startup/Shutdown TCL see the FME user documentation: FME Fundementals: FME Configuration > FME_BEGIN_TCL and FME_END_TCL > FME_BEGIN_TCL

For example to delete an Excel file before writing the script would look something like:

puts "***Startup TCL: Deleting file: $FME_MacroValues(DestDataset_XLS_ADO)"
file delete $FME_MacroValues(DestDataset_XLS_ADO)

The 'puts' will log a message in the FME log file.