It is simple. The value after the comma is the amount of the pickup
shift-M 6/8,2 (because you want two eighths)
shift-M 4/4,3 (because you want three quarters)
Or shift-M 4/4,1.5 if you want just three eighths.
Or strangely shift-M 4/4,6 (if you want six quarters in the pickup)