I’d go for a small piece of metal on the button and a switch of some sort to connect it to a large piece of metal nearby. It should work even better if that large metal part is near the metalwork of the washing machine.
The touchpad should provide all of the electric fields required. I don’t understand why you want to add a AA battery?
I should probably post a detailed explanation of how those capacitive buttons work when I’m somewhat less busy and back in front of my computer. And I’m sorry if things are not clear.
