Now that the chin rest behavior is consistent after practicing during the last week, change your criteria to focus on duration. Mark and reward for the initial chin rest and using multiple rewards given in a way that your dog can maintain the behavior if possible (small, soft treats work best for this), which will increase duration of the behavior itself. Rapid delivery of the rewards will help your dog be successful.
Once your puppy understands the game, slowly begin to increase the amount of time between the rewards so that your dog is holding the chin rest for longer and longer.
Once your dog is holding the chin rest for at least 5 seconds, go ahead and add a verbal cue if you desire. Your hand presented for the chin rest is a cue already, but some people prefer to add an additional verbal cue. Give the verbal cue and then present your hand for the chin rest. This way your dog will begin to connect the verbal cue as a prediction of the opportunity to perform the behavior.