test.ascx

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

<script runat="server"  >
  
  public event EventHandler Change;
  
  protected void OnChange() {
    if (Change != null) {
      Change(this,new EventArgs());
    }
  }
  
  void OnMyButtonClick(object sender, EventArgs e) {
    OnChange();
  }
  
</script>

<h1>
  Some Information <asp:LinkButton runat="server" 
    OnCLick="OnMyButtonClick">Click Me</asp:LinkButton>
</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.Change += new EventHandler(OnMyControlChange);
  }
  
  void OnMyControlChange(object sender, EventArgs e) {
    _myLabel.Text = "User Control Changed";
  }
</script>


<form runat="server" id="_myForm">
  <uc:myControl id="_myControl" runat="server"/>
  Some Additional information.<br>
  <asp:Label id="_myLabel" runat="server"/>
</form>