I'm currently trying out the Notification Subscription to track LocationUpdate events.
The CMX API document is the https://cmxlocationsandbox.cisco.com/apidocs, and is updated with each release.
The latest CMX REST API Guide is the https://www.cisco.com/c/en/us/td/docs/wireless/mse/10-3/api/b_cmx_103_api_reference/configuration.html
CMX REST API Getting Started Guide is the https://www.cisco.com/c/en/us/td/docs/wireless/mse/10-2/api/b_cg_CMX_REST_API_Getting_Started_Guide/b_cg_CMX_REST_API_Getting_Started_Guide_chapter_01.html
Here's a way to determine the REST API body for CMX notification subscriptions. Create the Notification in the CMX graphical user interface, login to https://cmxlocationsandbox.cisco.com with learning/learning, create a new notification under MANAGE --> Notifications, then use the CMX REST API GET "notifications by name" to see how the body was formatted.
In the GUI under Manage click on Notifications, provide the details for the Notification and Save.
Then the REST API returns the notification you just created by name:
Finally, delete the original notification:
and then re-create the REST API notification subscription with the response from GET above:
only allows twenty characters. Also, be careful of naming convention, using no spaces or special characters, to be safe.