title: How/Why callback url is used with respect to API?
body: <p>I am using HelloSign api for c# and on hellosign API page they are asking me to add callback url and they will test it. I am not sure what is call back url and where and how to use it. Can any one please explain me what is call back and why its useful or how to use? 
  Or can you explain call back url in simple language?</p>