It’s not recognized by Alexa.
It doesn’t accept commands, so you can’t have either the ST app or Alexa “push” it for you.
If you want Alexa to do something when the button is pushed, you have to create a virtual switch or sensor, create an ST automation to have the button being pressed turn on the virtual device, and then you can trigger an echo routine from that virtual device being activated.
FAQ: Can I trigger an Echo Action without Speaking to It?
What exactly were you trying to accomplish? Or did you just wonder why it didn’t show up?