as title says, there is no option to add a authenticator under my accounts settings. I believe it is because i already had one, then switched to the new jagex launcher but there isnt any setting related to it under my account.
If you already have Authenticator then you don't need to set up another one just because you are now using the Launcher and have added your accounts to it.
However, once you upgrade to the new 'Jagex Account' you
will
need to set up a new Auth and your current ones will become unneeded.
Click on 'My Account' at the top of this page then click on the blue 'Jagex Account Management' button on the next screen.
The Authenticator setting options are in there.