You're almost there. Go to the inputbox properties, Constraints, Listed Values, but set instead:
=Concat(DISTINCT Field, ',')
That will return a comma separated list of all distinct values for field "Field". Note that the comma may not be your list separator, to know which one will work if that doesn't, use Ctrl + Shift + Q and search for "ListSep", and you will see something like this in the dialog that shows up:
ListSep Doc=";"(3B) Sys=";"(3B)
The "Doc" is the one that matters, so in my case, the separator in that Concat() function should be ";" instead of the comma. In this regard, check (and vote if you feel like) this idea so you as developer can choose the list separator instead of using the regional settings.
Anyway, once you get this working, make sure you check "Predefined Values in Drop Down" in the "Value List" section of the Constraints tab in the inputbox properties.
Hope that helps.