its because you should do it whit a get consult not a post an the url should be like this: https://graph.facebook.com/oauth/access_token?client_id={your app id}&client_secret={your app secret}&grant_type=fb_exchange_token&fb_exchange_token={your shor live token}

all the thing you need are in "app settings>basic" and you should not use a json to this consult