use codebehind and define a class for the usercontrol, for example
WebUserControl1.ascx
<%@ Control Language="vb" AutoEventWireup="false" Codebehind="WebUserControl1.ascx.vb" Inherits="WebUserControl1"%>
WebUserControl.ascx.vb;
Public Class WebUserControl1
Inherits System.Web.UI.UserControl
...
Public Property MyText() As String
Get
Return TextBox1.Text
End Get
Set(ByVal Value As String)
TextBox1.Text = Value
End Set
End Property