It'll be here:
Make sure if any 0 (zero) or O (letter O) in it you know the difference if not using the link.
What you might want to do, is click on the link, then save the URL to then supply the URL others when they're ready to activate. When they click on the URL, it'll take them directly to the activation screen with your referral code already pre-filled.
.... Or, simply write it down or screenshot it and provide it that way, works fine too if the person entering it pays attention if manually entering it.