Issue with Trello webhook

I'm currently working with Trello's API and I'm trying to set a webhook, but I'm running into issues that hopefully someone can help me resolve. First of all in the Trello API and docs (http://ift.tt/1wp8fHN), creating a webhook requires sending the idModel as part of the json. I guess my first problem is that I'm not sure exactly sure what the idModel, or how I can find this id. I tried to use the id I get if I get tokens, but currently I'm getting a 400 response error when I send a request to the following endpoint:

http://ift.tt/1ellhzM

and my json looks something like (replace callbackURL with my actual callback url):

{
 description: "My first webhook",
 callbackURL: callbackURL,
 idModel: "54fdef9430a36282010f6230",
}