How do I add items to the dishwasher after a cycle has already started?

You can add items anytime before the main wash starts by lifting the door latch to stop the cycle and waiting for the spraying action to stop before opening the door. If the detergent dispenser cover is still closed, you can add the item. Close the door firmly until it latches, then press START/RESUME — the dishwasher will resume after a 5-second pause.

From the official Whirlpool W10021620 Built-In Dishwasher manual, page 14

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