Retrieving Game Center username for RuneScape on PC

Hi, i made an account on my iphone for runescape with gamecenter, now i wanted to try and play on the pc but i can’t find my username cause gamecenter is not compatible with the game. Even if i try to use apple id it can’t find my username how can i fix this ?

[Re-Titled by Moderator]

Original Title: gamecenter and runescape