It is very simple, just add a property:
public string Value {
get { return textBox1.Text; }
set { textBox1.Text = value; }
}
Using the Text property is a bit trickier, the UserControl class intentionally hides it. You'll need to override the attributes to put it back in working order:
[Browsable(true), EditorBrowsable(EditorBrowsableState.Always), Bindable(true)]
[DesignerSerializationVisibility(DesignerSerializationVisibility.Visible)]
public override string Text {
get { return textBox1.Text; }
set { textBox1.Text = value; }
}