ADT Pulse has its own Alexa skill. I’m not sure what services you have to be subscribed for in order for it to work,
As far as I know, there is no way to directly access the ADT pulse security features from SmartThings.
What some people have done, which is kludgy but works, is to set up a second speaker device, typically an android phone, next to the echo device, and then automate the android device, usually using LANnouncer, to speak the appropriate command when you want. Then the echo will respond to that command, and the ADT pulse skill will kick in. However, this requires significant technical skills to set up.
Like I said, kludgy, but it will work as long as the ADT pulse skill works.