On the Energizer siren output, connect a 12volt relay to its positive and negative respectively.
Then on the relay use COM and NO connection to connect to RDC/SMS unit/radio - COM goes to positive on RDC and NO to input 1 on RDC.
Program input 1 on RDC using dongle.
So if the energizer siren activates it will simultaneous switch on the relay then activating the RDC or radio transmitter.