link apple tv to directv account

i have an Apple TV account that i want to link to my DirecTV account.  the DirecTV web site says "Already have Apple TV?  We’ve got you!  You can link your existing Apple TV account to your DIRECTV bill", but then fails to provide any instructions on how to do that.  on the account page there's only an option to purchase the Apple TV package, no option to simply link an existing account.  how do i link an existing Apple TV account?