Identify where I am and plot routes (Google Maps V3 with JS)

Question

I am trying to create a script in JS and HTML5. Need to solve the following problem:

The user opens the page and the map appears in a predefined place. Then he clicks on an input and enters its location, (the input is above the map), then he clicks on a search button, and at the same moment, a route is drawn from the point where it is, to the place that was already predefined previously. (ex: a company).

My code is like this:

var map;
var directionsService = new google.maps.DirectionsService();
var info = new google.maps.InfoWindow({maxWidth: 840});

var marker = new google.maps.Marker({
    title: 'AUAU MIAU PETSHOP',
    icon: 'marker.png',
    position: new google.maps.LatLng('-18.894756', '-48.263564')
});

function initialize() {
    var options = {
        zoom: 15,
        center: marker.position,
        mapTypeId: google.maps.MapTypeId.ROADMAP
    };

    map = new google.maps.Map($('#map_content')[0], options);

    marker.setMap(map);

    google.maps.event.addListener(marker, 'click', function() {
        info.setContent('Avenida Bias Fortes, 382 - Lourdes, Belo Horizonte - MG, 30170-010, Brasil');
        info.open(map, marker);
    }); 
}

$(document).ready(function() {
    $('#form_route').submit(function() {
        info.close();
        marker.setMap(null);

        var directionsDisplay = new google.maps.DirectionsRenderer();

        var request = {
            origin: $("#route_from").val(),
            destination: marker.position,
            travelMode: google.maps.DirectionsTravelMode.DRIVING
        };

        directionsService.route(request, function(response, status) {
            if (status == google.maps.DirectionsStatus.OK) {
                directionsDisplay.setDirections(response);
                directionsDisplay.setMap(map);
            }
        });

        return false;
    });
    
});

#map_content {
  height: 350px;
}

<script src="https://maps.googleapis.com/maps/api/js?v=3.exp"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.3/jquery.min.js"></script>

<form action="" method="post" id="form_route">
                                                    
  <label>Origem: 
    <input type="text" id="route_from" size="50" />
  </label>
  <input type="submit" value="Traçar rota" />
    
</form>
    
<div id="map_content"></div>

How can I adapt it to solve my problem?