Ho to Use __doPostBack

Add this code behind page_load method

        Page.GetPostBackEventReference(btnHidden);

Add this javascript code on your markup

function showWindowOnEdit()
        {   
            var button = document.getElementById('<%= btnHidden.ClientID %>');
            
           __doPostBack(button.name,"OnClick");
 }

And your done with your functionality. As simple as like this.