One button strobe ON/OFF toggle

Does anybody know how to do this?

I use a Sony A&RV, and I find myself switching from strobes to ambient fairly often.  Does anyone know a way to set this toggle on one of the function buttons? I know there's other ways, but I'm looking for a one button ON/OFF.

Gary

If you can't find flash on/off in the list of assignable functions for custom buttons then you have 3 recall positions on the mode dial, you could save one configuration for shooting with flash on and a second configuration with flash turned off and you could also have a different set of defaults to use for ambient shooting.