Add "rel"-attribute to img tag in TinyMCE 5 editor

I search a solution to add automatically a "rel-attribute" to every image which is added in the TinyMCE 5 editor. So the html tag should look like:

<img class="imageborder" src="https://xy.jpg" rel="lightbox" width="500" height="333" />

I've tried it like that, but it doesn't get added. This is the JSFiddle.

tinyMCE.init({
    selector: '#myTextarea',
    plugins: 'code image autolink link lists charmap print preview textcolor',
    toolbar: 'code image link | undo redo | insert | ',
    menubar: false,
    min_height: 300,
    image_class_list: [
        { title: 'imageborder', value: 'imageborder' },
    ],
    image_rel_list: [
        { title: 'lightbox', value: 'lightbox' },
    ],
    setup: function (ed) {
        ed.on("keyup", function () {
            $('#preview').html(tinymce.activeEditor.getContent());
        });
    }
});

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