Type /linkaccount or /link, and then, after that it should take you to a menu that allows you to choose which one to sync. It should be on the right, according to my memory (but don't trust my small peanut brain I have, I have terrible memory). Once you selected/clicked it, a link should pop up into your chat, that allows you to sync your accounts.
If that doesn't work, try /cow and clicking on the 'daily rewards' minecart chest in the middle of the screen. It will take you to pretty much the same menu. And if that, and that doesn't work just try /sync.