My working configuration for this method:
rescue.elf (Ulaunch 4.21)
When I start my PS2, FreeMCBoot loads up Ulaunchelf, I hit X to load Codebreaker.elf, I select cheats, press start, start the game, and ESR loads. THIS WILL WORK WITH ORIGINAL GAMES TOO! just make sure the original is in BEFORE you start the PS2.