I know you meant this, but just to avoid any confusion…
At the present time, it can’t be just a virtual switch, because Alexa routines don’t currently trigger from switches. Just sensors, locks, and Flic buttons.
It has to be a virtual device which is BOTH A sensor and a switch. When you turn on the switch, the sensor opens, and the sensor opening is what will trigger an Alexa routine.
Fortunately there are several community created devices of this type, so you can just use one of those. There’s even one which is an Edge Driver, so you don’t have to worry about compatibility with the new smartthings platform. 
See the community FAQ for step-by-step instructions: