You are sending the string value "ctcurrent" to ThingSpeak, in your querry string parameters (not the value of the variable). You do have it as a parameter in the JSON, but I suspect the syntax is wrong. I would not send it in the JSON. Also, you have your write api key as a parameter in the webhook.
It is possible to use webhooks, and its nice to be able to abstract parameters so you can reuse a webhook for multiple processes. But it can be a bit tricky. I would really reccomend theThingSpeak library I mentioned above. It does all the work for you and doesn't need a webhook.
If you are really stuck on using the webhook, I'd find and example and carefully copy what you see.
Also, did you export only field 1? Do you have field 2 enabled in your channel view? If you exported the whole feed but only had channel 1 enabled, you might see something like what you show above. I was expecting to see the string "interval" based on what you shared.