Try this.
Private Sub CommandButton2_Click() ' Browse & Select File
strPassword = "your password here"
If ActiveDocument.ProtectionType <> wdNoProtection Then
ActiveDocument.Unprotect Password:=strPassword
End If
With Application.FileDialog(msoFileDialogFilePicker)
.AllowMultiSelect = False
.Title = "Select the File that you want to insert"
If .Show = True Then
FiletoInsert = .SelectedItems(1)
Else
Exit Sub
End If
End With
Application.ActiveDocument.Protect wdAllowOnlyFormFields, Password:=strPassword
End Sub