Paste the example xml from the activity API into this and convert to json:
https://www.freeformatter.com/xml-to-json-converter.html
Copy the output of that and paste it into the left hand column of this:
Drill down on the right hand side to the data you want.
The path to the json data is at the top of the right hand coloumn. Replace x with value_json and you have the value template information for the restful sensor state.