I am trying to install my ecobee 3 light but the previous thermostat did not use any c wire. Fortunately, there was an unused wire in the wall and I would like to set it up as a c-wire. The problem is that I have no idea where to connect it to our boiler. We have a Burnham Series 2 boiler, which uses Intelligent Hydronic Control (Honeywell S9361A2072) and a Transformer (40 VA, Honeywell AT140D1297).
Is it possible to connect a c-wire to the boiler?
I did try to find information in the technical user manual, which includes the diagram below, but couldn't find anything that describes the different ports/slots. It does say that "For zone valve wiring, a separate 24V transformer is required rather than attempting to use the boiler mounted control". But I think that is unrelated.
Edit: Added image of P5-8.