A TinyMCE 4.* plugin to add support for Fonts.com fonts - place in a new folder at tinymce/plugins/fontsdotcom/ and replace src URL. Don't forget to add "fontsdotcom" to your TinyMCE "plugins" config array.

plugin.min.js

tinymce.PluginManager.add('fontsdotcom', function(editor) {
    editor.on('init', function() {

        // Get the DOM document object for the IFRAME
        var doc = this.getDoc();

        // Create a script element and insert the Fonts.com URL into it
        var script = doc.createElement("script");
        script.type = "text/javascript";
        script.src = "//fast.fonts.net/jsapi/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.js";

        // Add the script to the header
        doc.getElementsByTagName('head')[0].appendChild(script);
    });
});