How to create an automated input box.

I have the code below that will evaluate a string of characters. If
its less then 12 long it will trigger a message box if its equal it
will save the information and close the form.

I have this currectly on a button event, how could i place this same
logic say once i enter in my last character it will trigger the logic
written in the button control.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As
System.EventArgs) Handles btnOK.Click
If Len(Me.txtBarCode.Text) = 12 Then
_barcode = Me.txtBarCode.Text
Close()
Else
MessageBox.Show("The Current BarCode Scanned was not
correct, Please Re Enter")
Me.txtBarCode.Text = String.Empty
txtBarCode.Focus()
End If
End Sub
.