Pass topic to JSON data

[CAL37]

I created this flow so I could turn on the TV/change source between Hulu and Netflix using Alexa. I have the topic defined in the Alexa nodes and I can see the topic is being passed all the way to the call service node, but I don't know how to get it into the JSON data field to change the source. I was thinking I could put {{topic}}, but that is not working. What am I doing wrong?

{
"source": "{{topic}}"
}

[E1cid]

You are using JSONata not JSON.
What you are trying should work if you select JSON.
Or the J: expression would be
{"source": $$.topic}

[CAL37]

@E1cid I did not know there was a difference and just use the default J: expression. Thank you for your help!