Some basic functionality is missing with the Fluent UI combobox.
The Search text is not available as an option for use in other places. For example you can't set a variable or show it in a label
You also cannot set a DefaultSelectedItem. Example. The label on the right shows the value of varText. The bottom screenshot shows that Jerusalem is one of the values in the combobox. The DefaultSelectedItems property has been set to [varTest]. As you can see it is not populating a value into the combobox