How can I call a javascript function from inside a method?

public class bgchange : IMapServerDropDownBoxAction
{
    void IServerAction.ServerAction(ToolbarItemInfo info)
    {
     Some code...

[WebMethod]
public static void DoSome()
{

}

public class bgchange : IMapServerDropDownBoxAction
{

void IServerAction.ServerAction(ToolbarItemInfo info)
{
    ...my C# code to perform on dropdown selection...
        //now I want to trigger some javascript...
        // Define the name and type of the client scripts on the page.
        String csname1 = "PopupScript";
        Type cstype = this.GetType();

        // Get a ClientScriptManager reference from the Page class.
        ClientScriptManager cs = Page.ClientScript;

        // Check to see if the startup script is already registered.
        if (!cs.IsStartupScriptRegistered(cstype, csname1))
        {
            String cstext1 = "alert('Hello World');";
            cs.RegisterStartupScript(cstype, csname1, cstext1, true);
        }
}

<asp:ScriptManager ID="scriptManager1"
    runat="server" EnablePageMethods="true" />

Page.ClientScript.RegisterStartupScript(
    this.GetType(),
    "callDoSome",
    "PageMethods.DoSome(Callback_Function, null)",
    true);

<script language="javascript" type="text/javascript">
    function Callback_Function(result, context) {
        alert('WebMethod was called');
    }
</script>

public void ServerAction(ToolbarItemInfo info) {
    string jsfunction = "alert('Hello');";
    Map mapctrl = (Map)info.BuddyControls[0];
    CallbackResult cr = new CallbackResult(null, null, "javascript", jsfunction);
    mapctrl.CallbackResults.Add(cr);
}

string script = "<script language='javascript'>n";
script += "javascriptFunctionHere();n";
script += "</script>";

Page.RegisterStartupScript("ForceClientToCallServerMethod", script);

public partial class _Default : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        Page.ClientScript.RegisterStartupScript(
            this.GetType(),
            "helloworldpopup",
            "alert('hello world');",
            true);
    }
}