I know, but if you read the FAQ, you will see that smartthings allows you to create a virtual device which is both a sensor and a switch. It will literally look like two different devices to echo. But when you turn the switch on, the open sensor will send an “open” event, and when you turn the switch off, it will send a “close” event.
So now everything becomes pretty straightforward. And anything that you can do in SmartThings can be used to trigger an echo routine. Not just sensor events.
create one of these special dual function virtual devices. The instructions and details are in the FAQ already linked to above.
set up a SmartThings automation so that the virtual switch part of that device Turns on/off when the real physical switch turns on. ( most people would just use the “mirror” option in the official smart lights feature.
create an echo routine (not a SmartThings routine) based on the virtual sensor part of the device from step one.
And that’s all you need. Now turning on the physical switch triggers the echo routine. Ta dah!
And here is the FAQ link again.
FAQ: Can I trigger an Echo Action without Speaking to It?