how to pass a list of list documents to custom function

Hi - I have a custom function that takes as input a list of user data type field called AvailableSlotsStruct which itself has a list so its a nested list.

List<AvailableSlotsStruct>? listAvSlotsByDat. AvailableSlotsStruct further contains a list.

So the custom function takes the above as input and returns a string. My goal is to call this function and set a text field with the return value. However I am not able to set up a call to this function using flutterflow UI as shown below. I am confused with options in the UI such as "map list items" etc as my goal is to pass the entire collection of this list<list> that i have queried and kept in an action output variable.