How to use JSON in Web Hooks message body?

I'm having an issue where the data aspect (Message Body), of the web hook's REST request is not being called/recognized. I was wondering if I'm using JSON in the message body correctly.
I'm using it like this (this is what I'm putting in the Message body):
{
"Key1": "Value1",
"Key2": "Value2",
"Key3": "Value3"
}