Adding the c wire to a two wire system
You'll either need to use a separate 24VAC transformer, or replace the boiler relay with something that provides a C terminal
Sadly, your Argo AR821, like many older boiler relays, does not provide a C terminal at all.  As a result, you'll need to either wire an external 24VAC transformer to Rc and C on the thermostat to provide it with power while connecting Rh and W to the relay terminals, or replace your boiler relay with a newer model that provides a C terminal.