I'll confess up front that I don't know how to use the Word "PRINT" field, but if that's a way of passing thru PostScript code:
Device-specific commands are handled by the "setpagedevice" PostScript operator.
It can get tricky, depending on your setup. For example, you may be able to assing paper color "pink" to a specific tray, then you would use the MediaColor dictionary value "pink" and let the printer pick the tray:
<< /PageSize [612 792]
/MediaColor (pink)
>> setpagedevice
Sometimes the printer numbers its trays, in which case you can try the MediaPosition entry:
<< /PageSize [612 792]
/MediaPosition 1
>> setpagedevice
Then too, paper size alone may cause the printer to switch trays. So specifying /PageSize [612 792] vs. /PageSize [792 612] may be enough to do the trick.
You'll just have to experiment!