And is the assignment conditional based on postback?
If Not IsPostback Then
MyBox.Text = "Default value"
End If
If it doesn't look like this, it will set the "default" value every time the page loads which will overwrite any posted changed.
(BTW- I guess we are talking about ASP.NET here. You didn't specify web forms or win forms and I didn't even notice the form was just
VB.NET. When you post a question in this form, it's helpful to specify detail about the environment and application you are working in where appropriate.)