Radcombobox onitemsrequested event not calling javascript function from code behind

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C#

ASP.NET

Telerik

radcombobox

Hi All,
I am stuck while calling javascript function from code behind on OnItemsRequested event. somehow I can call same js from OnTextChanged event.
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below is my code please help-

asp.net -

<telerik:RadComboBox ID="ddl1" runat="server" Height="150" OnClientItemsRequesting="OnClientRequestingAC" OnItemsRequested="ddl1_ItemRequested"
                                                    EmptyMessage="" EnableLoadOnDemand="true" ShowMoreResultsBox="false" EnableVirtualScrolling="false"
                                                    AutoPostBack="true" MarkFirstMatch="false" ShowDropDownOnTextboxClick="false" Style="width: 200px; height: 20px !important;"
                                                    MinFilterLength="3" ForeColor="Black" ShowToggleImage="true" 
                                                    ToolTip="Search by inputing number. Press Enter to select." LoadingMessage="Loading matching records..."                                                    OnClientItemsRequestFailed="OnClientACRequestFailedHandler" OnTextChanged="ddl1_TextChanged"  OnSelectedIndexChanged="ddl1_SelectedIndexChanged" onkeyup="return CallMethod(event,this.id);" >

in server side code-

Protected Sub ddl1_ItemRequested(sender As Object, e As RadComboBoxItemsRequestedEventArgs)
 
'' Doing some data fetching and JSON request preparation here...

      System.Web.UI.ScriptManager.RegisterStartupScript(Me.Page, Me.GetType(), "SearchRequestedJS", "GetSearchQueryData('" + JSONrequest + "');", True)

Catch ex As Exception
End Try
    End Sub

What I have tried:

I've tried using ScriptManager.RegisterClientScriptBlock but no luck. Am I missing something?