Off topic for this thread, but you need to create a virtual device that looks like both a switch and either a motion or contact sensor (because Alexa can only trigger routines from either a motion or contact sensor, and ST can only change the state of a virtual switch automatically). Several ways to do this, here’s one:
https://community.smartthings.com/t/release-alexa-virtual-device-creator-formally-alexa-virtual-switch-creator/103318