Ok. Try copying the code I provided in the post above and pasting that code in the "On Current" event of your form.
If you are not familiar with doing this, First, open your form in design view. Then be sure you have the form selected (not the listbox or any other control). In design view there is a square just below the title bar and above the ruller bar on the left. Click that square to be sure the form is selected. A small black square should appear when you have it selected.
Next, open the Properties window and click on the "Event" tab. The On Current event should be the first event listed. Double click in the space at the right of the "On current". You should see "[Event Procedure]" appear in that space. Then click on the Button with the 3 dots at the right end of this property.
The VBA code window will be presented with the following in it:
Option Compare Database
Option Explicit
Private Sub Form_Current()
End Sub
Just paste the code in the space between the "Private Sub Form_Current" and the "End Sub" lines.
When you have pasted the code, the VBA code window should look like this:
Option Compare Database
Option Explicit
Private Sub Form_Current()
If IsNull (Me.NameOfYourTextBox) then
Me.NameOfYourTextBox = Me.NameOfYourTextBox.Itemdata(0)
End If
End Sub
Now, just replace the "NameOfYourTextBox" with the actual name of your listbox control.
Open your form in form view and see if the first item is now selected.