The same issue applies to https://www.wikifunctions.org/view/en/Z6804 and when populating a list with Z6004 as its type. In these contexts, there is no option to provide the reference by means of a call to a different function.

In an untyped (Z1) list, the implicit Z6824 function call is exposed, so the argument can be entered. But in a context where the list can be saved, changing the list type from Z1 to Z6004 results in the deletion of such function calls. (See T377802.)

Wikifunctions currently has a "lexeme selector", which shows up when composing a call to a function having an argument of type Wikidata lexeme or Wikidata lexeme reference. There will soon be a similar selector for Wikidata lexeme form (& reference) types; work has recently started on it, for T377825. Also, we are preparing a new wiki page that gives a general overview of Wikidata content support, as well as status regarding what's done and what's planned. We apologize for not publishing that info previously!

This now works as expected.