This is just another application of a targeting behavior where all 4 feet end up in the car or a crate. A handy application of things you have already taught.
Being able to send your dog to the car to “load up” is just another targeting behavior. You may want this behavior to be specific to loading into their crate in the car, or just loading into the car in general (or both) and you simply can train what you prefer.
Be sure to train an implied stay so that your dog does not leave the open crate or car until you release them.
Teach your puppy to load into their crate/car (even if they can’t be allowed to jump in yet).
For a dog who is already crate trained, this is a simple additional step to be able to invite your dog to get into the car on command.
If your dog is not already crate trained - see the videos about building value for going into and resting in the crate.
Adding the “load up” command is simply a matter of giving your dog an opportunity to try something with the car door open (or crate door open). If they choose to move toward the door, throw a treat inside the car or crate and continue to reinforce them by tossing more cookies in while you add distance, duration and distraction to the implied stay.
Release with your release word, and let them offer to hop in again. When they are happy to do so, you can label the behavior.