Cannot change Callback URL

I am trying to setup a OAuth2 Authentication but it will not let me alter the Callback URL. Without being able to edit this, my attempt to Get New Access Token fails. The information states that you cannot alter the URL when authenticating via browser, but I do not see an option to turn this off or on. When watching YouTube videos showing me how to test this particular API/Webservice, the videos show a Checkbox for this, but it does not seem to exist. Please advise!