Using a Google Apps API Key with Distance Matrix

I am using the Google Distance Mapping API and the documentation says I need an API key (but I can use it without it.) I would like to be able to control the usage, but I don't understand how to set it up.

I have a valid browser app API key from the Google Developers Console, it's new, so I'm assuming it's a version 3 key.

I have added valid links in the console

I have <script src="https://maps.googleapis.com/maps/api/js?v=3.exp"></script>

on my page

I am using code like this

function callback(response, status) {
    if (status!==google.maps.DistanceMatrixStatus.OK) {
        _googleError('Error was: ' + status);
    } else {
        var origins = response.originAddresses;

        for (var i = 0; i < origins.length; i++) {
            var results = response.rows[i].elements;
            for (var j = 0; j < results.length; j++) {
                    $("#calcDistance").val(results[j].distance.text);
               //Other stuff that works here
            }
        }
    }
}

function calculateDistances(start, end) {
    var service = new google.maps.DistanceMatrixService();
    service.getDistanceMatrix(
        {
            origins: [start],
            destinations: [end],
            travelMode: google.maps.TravelMode.DRIVING,
            unitSystem: google.maps.UnitSystem.IMPERIAL,
            avoidHighways: false,
            avoidTolls: false
        }, callback);
}

      

        
        
        
      

    

How everything works fine. When I try to add a key to things, head south. I tried

<script src="https://maps.googleapis.com/maps/api/js?key={MY_KEY}&v=3.exp"></script>

and

<script src="https://maps.googleapis.com/maps/api/js?key={MY_KEY}"></script>

bad luck. When I do any of these, I get an "Invalid URL" error, similar to this question .

I also tried adding key: {MY_KEY},

in calculateDistances()

- no luck with that.

Am I missing something obvious? (I feel like I)

UPDATE:

@ Dr.Molle's answer gave me what I was looking for. I have enabled "Google Maps JavaScript API v3" and changed <script src="https://maps.googleapis.com/maps/api/js?v=3.exp"></script>

to <script src="https://maps.googleapis.com/maps/api/js?key={MY_KEY}&v=3.exp"></script>

Now I can view the activity in the developer console as I would like.