This is an idea from G4IZH, his original article can be seen here  

Please note he states clearly:-

This modification requires the fitting of a resistor, diode and capacitor inside the IC-7300 and unless you are confident in doing so “do not attempt it”  I accept no responsibility for your mistakes while doing this.

The same applies here, you modify your rig, and build and use the switch at your own risk. It is your responsibility to ensure all precautions are taken.

I have made some very minor component changes, routed the wires in the rig differently, made a PCB, and added two switches to enable the unit to be manually switched if used on an unmodified IC 7300

Please refer to G4IZH’s site for details, the photos below show my build.