I don’t know why this option disappears when switched to dynamic choices, this is really an important feature.
On the Searchbox Element, the option of “Allow Entries not in list” is available in both static and Dynamic.
This feature is really important because its useful when you want to add the “typed” value as a new thing on the fly, so when a user types a new value, you can use a “When input value has changed” Event, and use the “Typed” value to create a new thing.
Please put this on you list.
(Also the “this input should not be empty” option would be helpful too)