function AddComplaint(a, b) {
    var c = {};
    c[dle_act_lang[3]] = function() {
        $(this).dialog("close")
    };
    c[dle_p_send] = function() {
        if (1 > $("#dle-promt-text").val().length) $("#dle-promt-text").addClass("ui-state-error");
        else {
            var c = $("#dle-promt-text").val();
            $(this).dialog("close");
            $("#dlepopup").remove();
            $.post(dle_root + "engine/ajax/complaint.php", {
                id: a,
                text: c,
                action: b
            }, function(a) {
                "ok" == a ? DLEalert(dle_p_send_ok, dle_info) : DLEalert(a, dle_info)
            })
        }
    };
    $("#dlepopup").remove();
    $("body").append("<div id='dlepopup' title='" + dle_complaint + "' style='display:none'><br /><textarea name='dle-promt-text' id='dle-promt-text' class='ui-widget-content ui-corner-all' style='width:97%;height:100px; padding: .4em;'></textarea></div>");
    $("#dlepopup").dialog({
        autoOpen: !0,
        width: 500,
        resizable: !1,
        dialogClass: "modalfixed",
        buttons: c
    });
    $(".modalfixed.ui-dialog").css({
        position: "fixed"
    });
    $("#dlepopup").dialog("option", "position", ["0", "0"])
}

 <button type="button" class="btn btn-info btn-lg" data-toggle="modal" data-target="#myModal">Open Modal</button>

  <!-- Modal -->
  <div class="modal fade" id="myModal" role="dialog">
    <div class="modal-dialog">
    
      <!-- Modal content-->
      <div class="modal-content">
        <div class="modal-header">
          <button type="button" class="close" data-dismiss="modal">&times;</button>
          <h4 class="modal-title">Modal Header</h4>
        </div>
        <div class="modal-body">
          <p>Some text in the modal.</p>
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
        </div>
      </div>
      
    </div>
  </div>