__doPostback from Javascript gives wrong Argument

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ASP.NET

Dear All,
I am using bellow code to postback from JavaScript but it seems I am not getting the argument i pass.

public string postBackStr; //Defined in class

postBackStr = Page.ClientScript.GetPostBackEventReference(this, "myArgument"); //Defined in Page Load

textBox = cmbCo.FindControl("TextBox") as TextBox;
if (textBox != null)
{textBox.Attributes.Add("onblur", "return getMinistryCode(this);"); // This is In Render
}

In javascript i have the function

function getMinistryCode(txt) {
var cmbSDURN = document.getElementById(document.getElementById('<%= this.cmbSDURN.ClientID %>').id + "_HiddenField");
if (cmbSDURN.value == 0) {
   alert('Please Select SDURN');
   <%= postBackStr %>
 }
 }

The postback occurs but i am getting cmbCo ClientID, let me mention that cmbCo is Ajax 3.5 ComboBox and I am assigning onblur.

And the code for argument value is

if (Page.IsPostBack)
        {
            string arg = Request["__EVENTTARGET"].ToString();
            Response.Write(arg);
}

As i stated the arg return the clientID of cmbCo, how can i get the argument which i am passing through Client Script?

I found the solution by bellow.

if (Page.IsPostBack)
        {
            string arg = Request["__EVENTTARGET"].ToString();
            if (arg.Equals(cmbSDURN.ClientID.Replace('_', '$')))
            {