Found out how to do it:
<head>
<script type="text/javascript" src="TinyMCE/tinymce.min.js"></script>
<script type="text/javascript">
tinyMCE.init({
plugins: [ 'fullscreen' ],
setup: function(editor) {
editor.on('init', function(e) {
editor.execCommand('mceFullScreen');
});
}
});
</script>
</head>
<body>
<textarea id="editor"></textarea>
</body>