Many of the online instructions for this refer to the Wix editor which is now termed the old editor as it has fundamentally changed ...
I haven't tested the following out, so if any of these works for you let me know via a comment.
In this 2015 link, the Wix support team reference a specific landing page menu option:
here, here (using a splash video), here (using a button workaround) and here.
Bear in mind that you can request technical help from Wix directly!