Display Latitude and Longitude Coordinates in TextBox when Google Map is clicked using JavaScript

Here I have created smaple that full-fill your requirement.

HTML

<div>
    <script type="text/javascript" src="http://maps.googleapis.com/maps/api/js?sensor=false"></script>
    <script type="text/javascript">
        window.onload = function () {
            if (navigator.geolocation) {
                navigator.geolocation.getCurrentPosition(function (position) {
                    var latitude = position.coords.latitude;
                    var longitude = position.coords.longitude;
                    document.getElementById("<%=txtCoordinate.ClientID %>").value = latitude + ',' + longitude;
                    var mapOptions = {
                        center: new google.maps.LatLng(latitude, longitude),
                        zoom: 14,
                        mapTypeId: google.maps.MapTypeId.ROADMAP
                    };
                    var infoWindow = new google.maps.InfoWindow();
                    var latlngbounds = new google.maps.LatLngBounds();
                    var map = new google.maps.Map(document.getElementById("dvMap"), mapOptions);
                    google.maps.event.addListener(map, 'click', function (e) {
                        var latlng = new google.maps.LatLng(e.latLng.lat(), e.latLng.lng());
                        var geocoder = geocoder = new google.maps.Geocoder();
                        geocoder.geocode({ 'latLng': latlng }, function (results, status) {
                            if (status == google.maps.GeocoderStatus.OK) {
                                if (results[1]) {
                                    //alert("Location: " + results[1].formatted_address + "\r\nLatitude: " + e.latLng.lat() + "\r\nLongitude: " + e.latLng.lng());
                                    document.getElementById("<%=txtCoordinate.ClientID %>").value = e.latLng.lat() + ',' + e.latLng.lng();
                                }
                            }
                        });
                    });
                });
            } else {
                alert("Geolocation API is not supported in your browser.");
            }
        }
    </script>
    <div id="dvMap" style="width: 400px; height: 400px">
    </div>
    <br />
    <br />
    Coordinate :<asp:TextBox ID="txtCoordinate" runat="server" /><br />
</div>

Screenshot