Put the salmon fillets on a plate. Rub them with the mushroom soy sauce, top with the ginger, green onions, and garlic, and set aside at room temperature to marinate for about 20 minutes.
Add ¾ cup water to the rice cooker and bring to a boil.
Carefully transfer the salmon fillets to the steamer basket insert that came with the rice cooker. Close the lid again, set to cook 10 to 15 minutes depending on your taste.