In LibreOffice the drop-down list is called a "combo box." When you a create a form, the combo box draws a drop-down menu on a single line, from which users select from a series of preset values.

By default, the combo box will select the first item in the list that matches the user input, but it will continue to display all the other items as well. Our technique displays only matching items.

The Drop-Down List is to allow the reader to select one of the options you specify. The Combo Box includes the answers you specify, but also gives readers the opportunity to type their own option.