Have your helper approach and knock/ring the door again. When this happens, encourage your dog to run to their bed – ideally placed in a room where visitors wouldn’t enter - and reward them there by dropping a handful of treats onto their bed. Repeat this process several times during one session, but spend no longer than 10 minutes per session.
Over a number of sessions your dog will start to respond to the knock/bell by running to their bed with you. Encourage them to lead the way, turn it into a race to see who will get there first – it needs to be as fun as possible! When they get to their bed spend some time giving them a fuss and treats. The more you practice the better they will become, eventually they will run to bed all by themselves and wait for you to bring them a treat.
When you get to this stage, start to give a longer lasting treat, such as a stuffed interactive feeder, that will take them a good while to enjoy. You can now introduce short periods of closing the door to this room so that they are left alone to enjoy their treat.