How can you have a continuous siren/alarm on homebase and 2C cameras

Hi all

I have motion sensors in the house and 2 2c cameras externally. I can set an alarm or siren to go off on my homebase 2 and the external 2c cameras when I set to away mode or custom mode. But the alarm/siren ends after a few seconds. How can I set this to be a continuous alarm/siren. I can see no setting for this? Please help…

Thanks
H