It cant really be done to be frank. You have to have the code readily available and at some point, have to become core, then you put the code, let your core expire, and you get to keep the code that was put there to begin with. Anyone else who tells you there is a way to "Hack" around it, is lying.
Yeah, it would require paying at least once, posting, then expire. Please also note you cant add to it until you pay again. (Except for customising and no ads, the only real good thing about core is the live notifications, so then people dont have to wait to wonder if you got theyre messages. This is good if you are roleplaying, or getting a commission done where communication is critical.