Public class modifier for WPF control

I'm creating Windows application and Class library. Class library contains WPF control named "InsertForm.xaml"

InsertForm contains TextBox named eUserName.

I'm using the following code to show InsertForm. That's successful. But I can't access eUserName. How to set Textbox modifiers to public?

using System.Windows.Forms.Integration

ElementHost host = new ElementHost();
iform= new Extender.InsertForm();
host.Child = iform;
this.Controls.Add(host);