Find a Water Pump for Clothes Washer
Possibly a sump pump.

Or, you can get about 8 psi for every 20 feet of height of the tub, such as on the roof or in a tree.

Or you can put it on pause, open the door and pour in the water.

Or you can remove the top cover, and find the vent in the top of the outer stationary tub, attach a hose and fill.

You could get poly drum 20 gallons or larger, or gang together 4 five gallon buckets with gasket lids.  Attach a bicycle or auto tire valve stem and a garden hose outlet. Put your 15 gallons water into the drum. Replace the screw-bung so it's air tight then pressurize it bicycle pump.  The smaller the air space, the less time to pressurize, but the quicker the pressure will drop.