to keep the circuits isolated can use a 12VDC relay in the 12VDC circuit.
the ( potential free ) contacts of the relay you can use / connect like a push button
follow the manuals about reading a GPIO connected push button.
just to be clear the ground under the 12v is the ground for the 12v circuit not the pi.
you can simply leave the switch shown out of the circuit for you circuit.
Please only ask questions in the forum I will not answer questions sent to me directly via PM Remember we want information.......................no information no help The use of crystal balls & mind reading is not supported