I'm not really sure if your intended functionality could be reached with a single listbox. Maybe you need to combine a listbox or a textbox with buttons for ▲ ▼. Also an alternatively could be to use a multibox or a customized pivot:
To add a drop-down selection to an input box, go to Input Box Properties > Constraints > Value List > Select 'Predefined Values in Drop-down', > Deselect 'Enable Edit Expression Dialog' > Select 'Listed Value' > Enter values separated by a semi-colon
concat(distinct(fieldname), ',') - Use this syntax to call the all values of listbox in listedvalues of inputbox.