All patients on the CPAM service must be referred by their doctor.
You can talk to your local pharmacist about joining the service. The pharmacist can provide you with the necessary forms for your doctor to fill in to get you signed up. Your doctor will always have final responsibility for your treatment, the pharmacist is just acting to assist them. From time to time, the pharmacist may need to contact your doctor to get additional support when dosing more complex results.
Or you can talk directly with your doctor, who may know of pharmacies in the area that offer the CPAM service.