Clear default option when clicked in dropdown so user can type in custom text

First and foremost apologies for taking this long to reply.

Here is the same script I posted earlier but modified to satisfy your requirement.

Be advised that this script  is employing a  ternary (conditional) operation. As such, you will notice that I placed my for loop operation at the very top of the script before  the rest of the declared variables that will interact in the ternary operation. 

/*
CUSTOM FORMAT SCRIPT -- RUN FROM EACH COMBOBOX 
OR INVOKE AS A FUNCTION FROM EACH COMBOBOX 
*/

/////////////////////// DECLARE YOUR VARIABLES ////////////////////////

var f = event.target; f.defaultValue = "Select an Item or Enter a Location";

      {
        for (i=0; i < this.getField("hiddenBox").numItems; i++); 

        var itemsList = this.getField("hiddenBox").getItemAt(i, false);

        var values = this.getField("hiddenBox").insertItemAt(event.value, "", -1);

        var comboValues = event.target.insertItemAt(itemsList, "", -1);

      }
            
var comboStatus = (event.value !=="" 
                   && event.value !==" " 
                   && event.value !=="Virtual" 
                   && event.value !=="Select an Item or Enter a Location") ? ([

                   
///////////// GET INDEXED FACE VALUES FROM HIDDEN FIELD ////////////////

                   this.getField("hiddenBox").insertItemAt(event.value, "", -1),
                   event.target.insertItemAt(itemsList, "", -1)

                   ]) : ([

                   f, 

                   event.target.display = display.noPrint
                   
                   ]);


////////////////// EXECUTE IF / ELSE CONDITION ////////////////////////

(event.value =="") ? ([ event.value = "Select an Item or Enter a Location",
                        event.target.display = display.noPrint,
                        this.getField("hiddenBox").value = "Select an Item or Enter a Location",

                        this.getField("hiddenBox").setItems([ 

                                      ["Select an Item or Enter a Location"], 

                                      [" "],
 
                                      ["Virtual"]

                                      ]) 

                                   ]) : comboStatus

I tested this script with 20 different dropdown menus and it doesn't require a "parent.child" naming convention for each of them.

This custom format script  also employs the use of a dropdown menu as the hidden field instead of a textfield ; it works with a hidden listbox as well, and you may also use it as custom calculation script.

The reason why I preferred this approach, simply because you can hold an entire list of indexed values in a combobox, while in the case of  updating values back and forth with a textfield  to many comboboxes requires a much more different approach (specially if using export values in those comboboxes)..

NOTE: It may also work as custom keystroke or mouse event action with a few modifications. .

Nevertheless, since you're not using Thom Parker methods for a MasterList that stays scured as a document-level script, I still employed some of his advised techniques that are found in the link I posted for you in my first reply.

All of the answers you need are there: Part 1 and Part2 of that article should you decide to use/modify  this script as a dynamic  MasterList..

This script update each of the comboboxes dynamically and without the need of of document-level functions nor validating other scripts in conjunction with custom keystroke scripts.

NOTE: You may need to work around on how to effectively reset the fields using clearItems() and the setItems() methods.

Let us know if this help.