Follow these steps:

1. Inside a Interface Editor, select the background.
   
2. Click in "+" at the Actions tab.
   

3. Note the two triggers available.
   

   And the possible actions after these triggers.
    

4. You will need to fill the URL, the element/attribute, the target value to look for to set the trigger, and set the reload period.
    
5. This is an similar example for a JSON file located in my computer, where we look wether the value of the attribute "found" changed to true, in order to go to a different scene.
   
6. Check the example JSON file attached at the end of this article.
   
7. Playback your template, and test!

Download the example JSON file in the link below.