Hy,

FK type in form:
- Combo type : working, default
- Modal Picker: working
- Radio: working
- List: not working, result viewing default combo type.

My problem:
I want to select Joomla users (multiselect) for an item, not just one piece. Multiple Selection of List Types?

Edit:
How to one item select multi users?

Try your combo with size="4", It may transform it to a list with 4 rows

muti-selectable lists are handled by the "relation" field

Have a look here : www.j-cook.pro/index.php/m/many-to-many