How to add URL to list using JavaScript

I have a list with a title and a url. I'm trying to add a new url using this function

function addListItem(url, listname, metadata, success, failure) {

    // Prepping our update
    var item = $.extend({
        "__metadata": { "type": getListItemType(listname)}
    }, metadata);

    // Executing our add
    $.ajax({
        url: url + "/_api/web/lists/getbytitle('" + listname + "')/items",
        type: "POST",
        contentType: "application/json;odata=verbose",
        data: JSON.stringify(item),
        headers: {
            "Accept": "application/json;odata=verbose",
            "X-RequestDigest": $("#__REQUESTDIGEST").val()
        },
        success: function (data) {
            success(data); // Returns the newly created list item information
        },
        error: function (data) {
            failure(data);
        }
    });

}

I can set the title using this metadata item:

metadata = { Title: 'myTitle' }

But how can I set the url in this context? This doesn't work:

metadata = { Title: 'myTitle', Url: 'http://myspurl' }