Ok then this might help. BTW - my use case is to ask Siri if we closed the garage door. I have a garagelinc controlling the main door via an ISY and its REST interface. In the shortcut I do the following:
1. create a URL (this is the long URL you can grab from the 'Information' tab on my.isy.io. In my case, that URL is the command to 'query' the garagelinc device (/rest/status/[device id].
2. create a text file that contains username:password (don't forget the colon)
3. Encode that text file with base64
4. Get contents of URL. In the 'headers' section add an 'Authorization' header key and then 'Basic' in the text section. After the word 'Basic' add your Base64 Encoded file
5. Set the name of 'Contents of URL' to output.txt (or something to your preference).
If you 'play' the shortcut right here - you will see the XML file output. Progress!
Now you have a text response from your query to the ISY REST API. The same format by the way is what you use to issue commands to devices or run programs. Its pretty straightforward from here.
If anyone else has some guidance on how you can take a value from the resulting XML/text file please share that. I'd like to know if the status of my GarageLinc is ON or OFF so I then might have Siri respond accordingly.
BTW - thanks to the other posters on this topic as its been via those that I've cobbled together the above.