I realized that membership numbers can be written in different ways.

US Choice membership numbers
GPX-ABC12345 or ABC12345

Canadian Choice membership numbers
CN-12345678901 or 12345678901

This is quite confusing to me. Which format do I need to use if I want to collect Choice points from a Choice partner (e.g. rental car or Topguest). Is it the first format (i.e. including the "GPX" or "CN") or is it the second format?