Something like below. Remember to set MultiSelect property for ListBox to "0 - fmMultiSelectSingle".
Code:
Private Sub CommandButton3_Click()
Dim iPlist As Integer
Dim nCont As Control
With Me.ListBox1
For iPlist = 0 To .ListCount - 1
If .Selected(iPlist) = True Then
For Each nCont In Me.Controls
If TypeName(nCont) = "TextBox" Then
If Len(nCont.Value) = 0 Then
nCont.Value = .List(iPlist)
Exit Sub
End If
End If
Next nCont
End If
Next iPlist
End With
End Sub