MAp sensor to 5v power momentary switching
 

I am in the beginning stages of installing a water injection kit, and want to be able to send 5v to the controller to say it's wide open throttle, spray full tilt. The controller I'm using (Devil's Own), has a 5v wire to power a standalone MAP sensor, so all I would need to do is switch from the normal voltage put out by the stock MAP sensor, to the 5v feed. I can't picture how this would work. A momentary switch would be what I want to use. Like a purge to cool off stuff. Any ideas on how to set this up?