How to retrieve objectId from Installation classes

i've been stuck from yesterday on how to retrieve objectId for currentInstallation from installation classes. i've been go through every inch about installation in iOS, rest API, javascript docs for parse. and im only able to retrieve objectid using this code. and i dont have any idea on how to set their chael when login.

// Store the deviceToken in the current installation and save it to Parse.
   PFInstallation *currentInstallation = [PFInstallation currentInstallation];
   //NSLog(@"%@", currentInstallation.objectId);

//Store the objectId for to var
   NSString *currentId = currentInstallation.objectId;
   //NSLog(@"%@", currentId);

But the problem is i have different user for my apps which is customer, staff and admin. and i want to specified which user will get the notification. and the purpose i ask for objectId is i want change their chael when they login. example, when customer make reservation, only staff can receive the notification.

Please help me with this one