Show alert or run JavaScript function after showing the appointment popup in DevExpress using VB.NET

Dim script As String = Nothing
Dim disablePopup As Boolean = false

Protected Sub ASPxScheduler1_AppointmentFormShowing(sender As Object, e As AppointmentFormEventArgs) Handles ASPxScheduler1.AppointmentFormShowing

        e.Container = New UserAppointmentFormTemplateContainer(CType(sender, ASPxScheduler))

        If disablePopup  Then
            script = "You do not have a permission to modify this appointment."
            e.Cancel = True
        Else
            script = Nothing
        End If

End Sub

    Protected Sub ASPxScheduler1_CustomJSProperties(sender As Object, e As DevExpress.Web.CustomJSPropertiesEventArgs) Handles ASPxScheduler1.CustomJSProperties

       e.Properties.Add("cpValue", script)

    End Sub

<dxwschs:ASPxScheduler ID="ASPxScheduler1" runat="server" ClientIDMode="AutoID" ClientInstanceName="scheduler" AppointmentDataSourceID="SqlDataSource1" Start="1753-01-01">
  <Storage>
    <Appointments AutoRetrieveId="True">
      <Mappings AllDay="AllDay" AppointmentId="UniqueID" Description="Description" End="EndDate" Label="Label" Location="Location" RecurrenceInfo="RecurrenceInfo" ReminderInfo="ReminderInfo" ResourceId="ResourceID" Start="StartDate" Status="Status" Subject="Subject" Type="Type" />
      <CustomFieldMappings>
        <dxwschs:ASPxAppointmentCustomFieldMapping Member="FilePath" Name="FilePath" />
        <dxwschs:ASPxAppointmentCustomFieldMapping Member="ApprovedImage" Name="ApprovedImage" />
        <dxwschs:ASPxAppointmentCustomFieldMapping Member="UserCompanyID" Name="UserCompanyID" />
      </CustomFieldMappings>
    </Appointments>
    <Resources>
      <Mappings Caption="ResourceName" Color="Color" Image="Image" ResourceId="ResourceID" />
    </Resources>
  </Storage>
  <Views>
    <DayView>
      <TimeRulers>
        <cc1:TimeRuler></cc1:TimeRuler>
      </TimeRulers>
      <Templates>

        <VerticalAppointmentTemplate>
          <VAppointmentTemplate:VerticalAppointmentTemplate runat="server" />
        </VerticalAppointmentTemplate>
      </Templates>
    </DayView>
    <WorkWeekView>
      <TimeRulers>
        <cc1:TimeRuler></cc1:TimeRuler>
      </TimeRulers>
      <Templates>
        <VerticalAppointmentTemplate>
          <VAppointmentTemplate:VerticalAppointmentTemplate runat="server" />
        </VerticalAppointmentTemplate>
      </Templates>
    </WorkWeekView>
    <WeekView Enabled="false"></WeekView>
    <FullWeekView Enabled="true">
      <TimeRulers>
        <cc1:TimeRuler></cc1:TimeRuler>
      </TimeRulers>
      <Templates>
        <VerticalAppointmentTemplate>
          <VAppointmentTemplate:VerticalAppointmentTemplate runat="server" />
        </VerticalAppointmentTemplate>
      </Templates>
    </FullWeekView>
  </Views>

  <ClientSideEvents EndCallback="function(s, e) { 
                                 if(scheduler.cpValue === null || scheduler.cpValue === undefined )
                                 {}else
                                 { alert(scheduler.cpValue);}
                                 }" />

  <OptionsCustomization AllowAppointmentDelete="None" AllowAppointmentDrag="None" />

  <OptionsForms AppointmentFormTemplateUrl="~/CustomSchedule/UserAppointmentForm.ascx" AppointmentInplaceEditorFormTemplateUrl="~/DevExpress/ASPxSchedulerForms/InplaceEditor.ascx" GotoDateFormTemplateUrl="~/DevExpress/ASPxSchedulerForms/GotoDateForm.ascx" RecurrentAppointmentDeleteFormTemplateUrl="~/DevExpress/ASPxSchedulerForms/RecurrentAppointmentDeleteForm.ascx" RecurrentAppointmentEditFormTemplateUrl="~/DevExpress/ASPxSchedulerForms/RecurrentAppointmentEditForm.ascx" RemindersFormTemplateUrl="~/DevExpress/ASPxSchedulerForms/ReminderForm.ascx" />
  <OptionsToolTips AppointmentDragToolTipUrl="~/DevExpress/ASPxSchedulerForms/AppointmentDragToolTip.ascx" AppointmentToolTipUrl="~/DevExpress/ASPxSchedulerForms/AppointmentToolTip.ascx" SelectionToolTipUrl="~/DevExpress/ASPxSchedulerForms/SelectionToolTip.ascx" />
</dxwschs:ASPxScheduler>