just connect the two wires of the Shelly Releais (OUT 1) to clamp C and 11 in parallel to the already clamped wires.

power the Shelly with a separate transformer at 5 

(I assume you should not take the power of the intercom in order not to disturb their logic of power levels)

now configure your shelly to your needs an you can trigger the opener from the Shelly Web UI