C-wire nest to burnham boiler

Connecting nest learning 3rd gen to burnham series 2 model 203 boiler. I am only using the system for heat. I have three wires at the thermostat R,W and an unused green that I would like to turn into a c-wire. How can I connect this wire to the boiler to become a c-wire? Option 2, Can I connect a c-wire adapter to the boiler end of the wire as to not draw power from the boiler? If so how?