While you can power on the module with the charger connected, COBRA does not recommend operating the unit on a shoot site with the charger connected. This is due to the charger's design to charge the battery at a defined voltage and current. Attempting to fire cues on the module connected to igniters may damage the module and likely not fire the actual igniters.
The 18M firing module does support external power options when equipped with the LiPo Rechargeable battery. For more information on using external power with your COBRA 18M please see the support document What types of external power does the 18M support?