I am not aware of any discount promo code provided by PMI for new PMI member, but you can save some money for PMP Exam fee when you are PMI member.
Also, they are many PMI approved online Trainers providing PMP Certification courses at a discounted prices with promo code if you are still looking for PMP courses.
It is highly unlikely that PMI will offer a "discount code" for professional membership dues or test fees. I've never seen it from any other professional organization that I belong to.
Training for Project Management Professional (PMP)®, PMI Agile Certified Practitioner (PMI-ACP)®, and Certified Associate in Project Management (CAPM)®