This could be accomplished with STHM, a virtual switch and an Alexa routine. Create a virtual switch with contact sensor using this Edge driver:
Add it to your STHM response, then create an Alexa routine with it as the trigger and the action could be whatever you want. Spoken alert, play a song, etc.
This can be set in STHM settings, but the UI isn’t very intuitive. If you turn off the toggle next to “use all motion sensors” and then tap the text you can select individual motion devices.