I have a grid where you can select items, a drop down and a button. You can select an action from the dropdown and the click the button to open the modal dialog.
I need to put the id of the selected items in the Session first (IMPORTANT!), and then I need to open a Modal Dialog from code behind. I have tried a few solutions but the client code always get executed before the server code.

Thanks in advance,


UPDATE

This is the code in the aspx page

    <script language="javascript" type="text/javascript">
    function ShowLightBox(pageUrl, lightBoxTitle, moreOptions) {
        options = {
            url: pageUrl,
            autoSize: true,
            title: lightBoxTitle
        }
        SP.UI.ModalDialog.showModalDialog(options);
        yPos = $(window.parent.document).scrollTop() + 20;
        $(".ms-dlgContent", window.parent.document).css({ position: 'fixed', top: yPos });
    }
    </script>


    <telerik:RadComboBox ID="rcActions" runat="server" 
                DataTextField="Key" DataValueField="Value">
            </telerik:RadComboBox>
            <asp:Button ID="btnGo" runat="server" Text="GO" OnClick="btnGo_OnClick" />
    
        <telerik:RadGrid ID="radGrid1" runat="server" AutoGenerateColumns="false" AllowPaging="true" OnNeedDataSource="radGrid1_OnNeedDataSource"    OnItemCommand="radGrid1_ItemCommand" OnColumnCreated="radGrid1_OnColumnCreated"
            OnItemDataBound="radGrid1_OnItemDataBound">
            <MasterTableView AutoGenerateColumns="false" NoMasterRecordsText="<%$Resources:GlobalResource, noRecords%>">
                <Columns>
                    <telerik:GridBoundColumn DataField="Id" UniqueName="Id" Visible="false">
                    </telerik:GridBoundColumn>
                    <telerik:GridBoundColumn DataField="UnitNumber" UniqueName="UnitNumber" Visible="false">
                    </telerik:GridBoundColumn>
                    <telerik:GridClientSelectColumn UniqueName="Selected">
                    </telerik:GridClientSelectColumn>
                </Columns>
                <NestedViewSettings>
                    <ParentTableRelation>
                        <telerik:GridRelationFields DetailKeyField="Id" MasterKeyField="Id" />
                    </ParentTableRelation>
                </NestedViewSettings>
                <NestedViewTemplate>
                    <asp:Panel runat="server" Visible="false" ID="InnerContainer">
                       <!-- Some code -->
                    </asp:Panel>
                </NestedViewTemplate>
            </MasterTableView>
        </telerik:RadGrid>
    
    This is the code behind
    
        protected void btnGo_OnClick(object sender, EventArgs e)
            {
                try
                {
                    var unitNumbers = new List<string>();
                    foreach (GridItem unit in radGrid1.SelectedItems)
                    {
                         var gridInfo = new GridInfo(unit);
                         unitNumbers.Add(gridInfo.UnitNumberCell.Text);
                    }
                    if (Session["UnitNumbers"] != null)
                         Session.Add("UnitNumbers", unitNumbers);
                     else
                        Session["UnitNumbers"] = unitNumbers;
    
                      // Here is where I want to put the code to open the lightbox, instead of       redirecting.
                      Response.Redirect("../Pages/MaintenanceDetails.aspx");
                }
                catch (Exception ex)
                {
                    
                }
            }

I have tried using
ScriptManager.RegisterStartupScript(GetType(),"hwa","ShowLightBox(param);",true); and it does not find the function.

Any other suggestion?