Payments tab; scroll down to the payee and select, hit Edit button top-right.
Then hit edit button against account you want to edit.
Populate reference, save.
This is on an Android device.
As for pulling through previous references, when you transfer to someone you will have the option to select a prior reference, if there is one, or put a new one in. Just press on the reference section.
It should do, I’ve just checked - once I go onto a payee and I have the screen to enter the amount I want to send, as soon as I click on the cell to fill in the reference a carousel of previously entered references show up for me?