IsCrossPagePostBack and find previous pages usercontrol value to another page

 if (Page.PreviousPage != null && Page.PreviousPage.IsCrossPagePostBack)
            {
                ContentPlaceHolder cph = (ContentPlaceHolder)Page.PreviousPage.Master.FindControl("MainContent");
                if (cph != null)
                {
                    UserControl ucViewLocateUs = (UserControl)cph.FindControl("ViewLocateUs1");
                    if (ucViewLocateUs != null)
                    {
                        DropDownList dpdListCity = ucViewLocateUs.FindControl("dpdListCity") as DropDownList;
                        FillAllLocateUsDetails(dpdListCity);
                    }
                }
            } 

Default button click on enter use javascript

<script type="text/javascript">
    if (document.addEventListener) {//if Firefox
        document.addEventListener("keypress", fireFoxHandler, true);
       
    } else {
       
        document.attachEvent("onkeyup", ieHandler);
    }
   
    function fireFoxHandler(evt) {
        if (evt.keyCode == 13) {
            document.getElementById("btnSave").click();
           
        }
    }

    function ieHandler(evt) {
        if (evt.keyCode == 13) {
            document.getElementById("btnSave").click();
        }
    }

</script>

<asp:Button ID="btnSave" ValidationGroup="Submit" runat="server" Text="Save" OnClick="btnSave_Click"
                    CssClass="save_btn" ClientIDMode="Static" />