How to get IEEE promotion code
A set of promotion codes often would be dedicated to the people, who are members of IEEE and has been promoted to the upper membership level, typically. Those members are supposed to present those codes to the other people, willing to become new members of IEEE. For example, an IEEE Member will receive a couple of promotion codes, as he/she has been promoted to IEEE Senior Member. He/She could grant those codes, as gift, to the others (especially his/her students), to be considered for the membership discount.

All in all, you better find any recently-promoted guy around yourself and if any, he/she would, probably, be able to provide you a promotion code, as you need.