Adding another sensor to my security configuration?

My ST + Alexa home security implementation was beast to install, but it has been working pretty well…
I have the ST hub, 3 sensors (which are installed on 3 doors) and a zwave alarm. These components are configured such that if the system is Armed on my iphone, if any of the sensors is tripped the alarm will go off, or if I can set them to disarm on iphone, then no alarm. I have integrated Alexa so that I can arm/disarm the whole system by talking to Alexa.
Now I want to add another sensor to a window, would anyone know how to go about this? I know I configured it in the first place, but it was with alot of help and Im sot sure where to get started. The new sensor should be live but I dont see it in the list of things on the app anywhere .Thanks

What security app are you using? Smart Home monitor?
Which hub are you using?
Which app are you using?