Using the Trains (F7) tool, click on the Edit Properties ("?") icon then click on the hopper to open up its properties window. If the creator of the hopper has designed it to take loads, then you will see one or more load dialogue sections in the window.
Once you have added the required load and if there are other identical wagons in the same consist that you want to give the same load, then click on the round button showing an arrow arching from one green car to two grey cars, this will copy the load to the other identical wagons in that consist.
