Looks like with the last change in NMX in a Formula Editor is not any more possible to refer to Lists. So the only way is to type them in. It's very time consuming and error prone. Could you please have a look if it can be changed.
It's a known bug.
Solved: Issue with formula editor :clicking on a list or l... - Anaplan Community
Workaround is: Either type manually or else switch back to Old Modelling Experience until bug is fixed
We have a fix for the issue and it will be released as part of the next large platform update currently scheduled for early March. Obviously this is subject to change but that's the current plan.
Is there any progress on this case?
It can be very frustrating trying to click through to list items and then having to type the whole string + reference out.