Not really a special tool for adjusting preload. Rather basic and crude operation in fact.

1. Raise bike UP so rear wheel is OFF ground. (This is a Must Do)
2. spray threaded upper area of shock with WD40 (or similar)

3. There are two rings on your shock (yes ... hard to get to) The top one is the "lock ring". Back off lock ring (IE unscrew UP and out of the way)

4. The lower ring is sat on the spring, that is the one you must shift in clockwise direction (tighten) to increase preload.

5. Best way to do it? I use a sturdy, 10" to 12" screw driver or pry bar of some kind. Using a hammer, gently tap the slots on the ring to move the ring around. This takes a while, don't get impatient and DON'T bash the threads on the shock! Takes about 20 minutes of hammering.

There are a few special tools for this but they usually don't work. So I just tap the ring round about 10 full revolutions. Should make a difference, but how much preload you add depends on your weight and luggage weight.

Check Sag before & after to gauge how much you've raised the bike UP.

Doing this will help the bike carrying a load but the standard shock is NOT GREAT .. it lacks rebound damping and you may not notice much difference other than the bike sitting a bit higher when parked.

Stock Suspension is really set up to handle a 150 lb. rider and perhaps 50 lb. of gear max. Above that and the bike may not be great riding off road or if pushed riding on road, may wallow. Off road, shock may bottom.

There are REAMS of feed back on suspension on the DR forum and every scenario ever imagined is explored. Here is a clever way to make your stand into an adjustable one. Any mechanic can do this: