How can I return a part of a list to a field on the screen

I am working in a C#  application

I have constituted  a List with the script Below:

StaffNameListBox.DataSource = GlobalConfig.Connection.GetEmployees_All();
StaffNameListBox.DisplayMember = "FullStaffName";

 public string FullStaffName
        {
            get
            {
                return $"{Staff_no}  { Surname } {Firname }";
            }
        }

(See  the Screen of The list below showing a record in the list

On the Click event on this record I want to return a part of the string (ie just the staff_no)

 to a text box on the screen.  The name of the textbox is StaffNoValue


My click event syntax  is as below:

  private void StaffNameListBox_Click(object sender, EventArgs e)
        {
            StaffNoValue.Text = StaffNameListBox.ValueMember;
        }

What I want Returned to the text box on screen is as shown below

StaffNoValue.Text = "N001"

How can I get a portion of a list returned into a field using the actual name with which the list was loaded

and not by searching for a sub string of the selected item

Thanks
Selection-From-List--11-01-2019.PNG