Attempt to generate ESRI REST Token generate 'no client_id specified' error even when client_id is in the request body

I'm making a HTTPS POST request to the following URL: http://ift.tt/1BmUsQV

In the Request body I've added the following JSON:

{"client_id":"<ID>","client_secret":"<Secret>","grant_type":"client_credentials","expiration":"1440","f":"json"}

Despite the client_id being clearly specified I always get the same response to my request:

{"error":{"code":400,"error":"invalid_request","error_description":"client_id not specified","message":"client_id not specified","details":[]}}

Is there something about the REST call I'm missing? Should I still be using JSON in the request body?