test.ascx

<%@ Control language="c#" %>

<script runat="server">
  public string Title {
    get {
      return _myLabel.Text;
    }
    set {
      _myLabel.Text = value;
    }
  }
</script>

<h1>
  <asp:Label runat="server" id="_myLabel"/>
</h1>
<hr>

test.aspx

<% @ Page language="c#" %>
<% @ Register tagprefix="uc" tagname="myControl" Src="test.ascx"%>

<script runat="server">
  void Page_Load(object sender, EventArgs e) {
    test_ascx MyControl = (test_ascx)FindControl("_myControl");
    MyControl.Title = "Some New Title!";
  }
</script>


<form runat="server" id="_myForm">
  <uc:myControl id="_myControl" runat="server" Title="Some Title!"/>
  Some Additional information.
</form>