Popup fired by a button - late response (clicking twice for postback of value)

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C#

ASP.NET

Javascript

I have attached a popup to an event handler of a button.

This is my client side code in javascript (asp.net):

<script type="text/javascript" language="javascript">
            String.Format = function () {
                var s = arguments[0];
                for (var i = 0; i < arguments.length - 1; i++) {
                    var reg = new RegExp("\\{" + i + "\\}", "gm");
                    s = s.replace(reg, arguments[i + 1]);
                }
                return s;
            };
            var dialogConfirmed = false;
    
            function SetDialogConfirmedFalse() {
                dialogConfirmed = false;
            }
    
            function ConfirmDialogInfo(widthX, obj, title, dialogText) {
                if (!dialogConfirmed) { //!$('#dialog').is(':data(dialog)')
                    $('body').append(String.Format("<div id='dialog' title='{0}'>{1}</div>",
                        title, dialogText));
    
                    $('#dialog').dialog({
                        height: 110,
                        width: widthX,
                        modal: true,
                        resizable: false,
                        draggable: false,
                        close: function (event, ui) { $('body').find('#dialog').remove(); },
                        buttons:
                        {
                            'Ok': function () {
                                $('#dialog').dialog('close');
                                dialogConfirmed = true;
                                if (obj) obj.click();
                            }
                        }
                    });
                    $(".ui-widget").css({ "font-size": +18 + "px" });
                }
                return dialogConfirmed;
            }
    
            function ConfirmDialog(obj, title, dialogText) {
                if (!dialogConfirmed) { //!$('#dialog').is(':data(dialog)')
                    $('body').append(String.Format("<div id='dialog' title='{0}'><p>{1}</p></div>",
                        title, dialogText));
    
                    $('#dialog').dialog({
                        height: 110,
                        modal: true,
                        resizable: false,
                        draggable: false,
                        close: function (event, ui) { $('body').find('#dialog').remove(); },
                        buttons:
                        {
                            'Ja': function () {
                                $('#dialog').dialog('close');
                                dialogConfirmed = true;
                                document.getElementById('<%= hdnTestValue.ClientID %>').value = dialogConfirmed;
                                //document.getElementById("hdnTestValue").value = dialogConfirmed;
                                //if (obj) obj.click();
                            },
                            'Nein': function () {
                                $('#dialog').dialog('close');
                                document.getElementById('<%= hdnTestValue.ClientID %>').value = dialogConfirmed;
                                //document.getElementById("hdnTestValue").value = dialogConfirmed;    
                            }
                        }
                    });
                    $(".ui-widget").css({ "font-size": +18 + "px" });
                }
            }
        </script>

    <asp:HiddenField  runat="server" ID="hdnTestValue" /> 

This is my server side code in C#:

protected void buttonStopCurrentContract_ButtonPressed(object sender, EventArgs e)
            {
                   
                        var standardMessage = "Do you really want to cancel this contract?";
    
                        var script = "ConfirmDialog('this', '" + CommonRes.Attention +
                                 "', '"+ standardMeldung + "');";
                        ScriptManager.RegisterStartupScript(this, GetType(), Guid.NewGuid().ToString(), script, true);
                        
                        // hdnTestValue is a hiddenfield
                        if (hdnTestValue.Value == "true")
                        {
                          // do something
                        }
                    
                
            }

When I run this code, ScriptManager.RegisterStartupScript waits till the end of the method and pops the dialog. After picked up a decision "yes" the decision is being saved in the hiddenfield. I have to click twice on the button to get into the if-condition. There is a postback problem. I have not much experience in javascript or asp. Can anybody give me advices / solutions?

Thank you in anticipation.