The Riot ID is the ID that will be used for purchases of Riot products. The ID is your username used in the game, however, you must have updated your account. If you haven't updated, just access the link:
If you haven't updated your account yet, you will need to do so on the page. If you have already done so, your ID is the username you used to log in to the page. It's worth remembering that the Riot ID is the same for all Riot games we offer. Select and recheck to ensure you are in the game that interests you.