So if I were to do 8 (a clue one), would I write #8CODE?
No, no. For the ones that required you to input a code, you just write whatever you think it is in ALL CAPS and without spacing or symbols.
But the CLUE eggs form #7 forward give you a code in the private message AFTER YOU'VE FOUND THEM, so you can keep track. Like, once you find egg 15, for example, the private message you get from the gyroid will have a code in it that starts with egg15.