OAuthInitiate_2.cls

public PageReference initiate(){
  String authorizeUrl = 
    'https://login.salesforce.com/services/oauth2/authorize';
  authorizeUrl += '?response_type=code';
  authorizeUrl += '&client_id=' + CLIENT_ID;
  authorizeUrl += '&redirect_uri=' + CALLBACK_URL;
  authorizeUrl += '&prompt=login consent';

  PageReference pref = new PageReference(authorizeUrl);
  pref.setRedirect(true);
  return pref;
}