Wait popup response before C# function

I'm trying to call a JavaScript function with a popup, and if user click on "Ok", it calls a C# function. But the page always PostBack in same time I am loading the JavaScript function.

My HTML ASP:Button :

<asp:Button ID="PrchBtn" runat="server" class="PrchBtn" Text="<%$ Resources:Resource, WebEDI_Save %>" OnClick="PrchBtn_Click" OnClientClick = "Confirm();" />

OnClientClick, it calls this JS function :

function Confirm() {
    var confirm_value = document.createElement("INPUT");
    confirm_value.type = "hidden";
    confirm_value.name = "confirm_value";

    alertify.confirm('<%= GetGlobalResourceObject("Resource","WebEDI_PDF_MsgBox") %>', function (e) {
        if (e) {
            confirm_value.value = "Yes";
        } else {
            confirm_value.value = "No";
        }
        document.forms[0].appendChild(confirm_value);
    });
}

And then my C# function :

public void PrchBtn_Click(object sender, EventArgs e)
    {
        //Code here...
    }

It's working with a simple "confirm" dialog. But I want to custom the popup that's why I use "Alertify" library.

Thanks for your help.

UPDATE (See comments #3) : By following this link (Call Code Behind Function from JavaScript (not AJAX!)) This is my actual code :

<asp:Button ID="PrchBtn" runat="server" class="PrchBtn" Text="<%$ Resources:Resource, WebEDI_Save %>" OnClientClick="Confirm(); return false;" />
<asp:Button runat="server" ID="PrchBtnHidden" ClientIDMode="Static" OnClick="PrchBtn_Click" style="display:none;" />

function Confirm() {
    var confirm_value = document.createElement("INPUT");
    confirm_value.type = "hidden";
    confirm_value.name = "confirm_value";

    alertify.confirm('<%= GetGlobalResourceObject("Resource","WebEDI_PDF_MsgBox") %>', function (e) {
        if (e) {
            confirm_value.value = "Yes";
            document.forms[0].appendChild(confirm_value);
            __doPostBack('<%= PrchBtnHidden.UniqueID %>');
        } else {
            confirm_value.value = "No";
            document.forms[0].appendChild(confirm_value);
            __doPostBack('<%= PrchBtnHidden.UniqueID %>');
        }
    });
}

But the problem is the same, JS and C# are doing stuff at the same time.

UPDATE (Bonus bug) : I don't know why but my alertify is bugged. On a prompt :

alertify.prompt("Message", function (e, str) {
        // str is the input text
        if (e) {
            Console.Log("Ok");
        } else {
            Console.Log("No");
        }
    }, "Default Value");

When I click on Ok or No, nothing is firing. And the TextBox content of the prompt is :

function (e, str) { // str is the input text if (e) { Console.Log("Ok"); } else { Console.Log("No"); } }

And with an Alertify.Confirm

alertify.confirm('<%= GetGlobalResourceObject("Resource","WebEDI_PDF_MsgBox") %>', function (e) {
        if (e) {
            confirm_value.value = "Yes";
            document.forms[0].appendChild(confirm_value);
            __doPostBack('<%= PrchBtnHidden.UniqueID %>');
        } else {
            confirm_value.value = "No";
            document.forms[0].appendChild(confirm_value);
            __doPostBack('<%= PrchBtnHidden.UniqueID %>');
        }
    });

Only "Ok" is firing. The cancel button does nothig.

SOLUTION : Took another alertify.js (http://alertifyjs.com/) And this is my JS function :

alertify.confirm('<%= GetGlobalResourceObject("Resource","WebEDI_PDF_MsgBox") %>', function () {
        confirm_value.value = "Yes";
        document.forms[0].appendChild(confirm_value);
        __doPostBack('<%= PrchBtnHidden.UniqueID %>');
    },
      function () {
          confirm_value.value = "No";
          document.forms[0].appendChild(confirm_value);
          __doPostBack('<%= PrchBtnHidden.UniqueID %>');
      }).set('labels', { ok: 'Ok', cancel: 'No' }); 

And it works !

Solution : Create 2 HTML buttons, one visible linked to the JavaScript function and another not visible linked to the C# method :

<asp:Button ID="PrchBtn" runat="server" class="PrchBtn" Text="<%$ Resources:Resource, WebEDI_Save %>" OnClientClick="Confirm(); return false;" />
<asp:Button runat="server" ID="PrchBtnHidden" ClientIDMode="Static" OnClick="PrchBtn_Click" style="display:none;" />

The JS :

function Confirm() {
    var confirm_value = document.createElement("INPUT");
    confirm_value.type = "hidden";
    confirm_value.name = "confirm_value";

    alertify.confirm('<%= GetGlobalResourceObject("Resource","WebEDI_PDF_MsgBox") %>', function () {
        confirm_value.value = "Yes";
        document.forms[0].appendChild(confirm_value);
        __doPostBack('<%= PrchBtnHidden.UniqueID %>');
    },
      function () {
          confirm_value.value = "No";
          document.forms[0].appendChild(confirm_value);
          __doPostBack('<%= PrchBtnHidden.UniqueID %>');
      }).set('labels', { ok: 'Ok', cancel: 'No' });
}

When you will click on the 1st button, it will call JS. Then JS will call the PostBack of the second button. And I had a problem with Alertify, so I used another source : http://alertifyjs.com/