Common.js

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mw.hook( 'wikipage.editform' ).add( function () {
    mw.loader.using( 'mediawiki.toolbar' ).then( function () {
        mw.toolbar.addButton( {
            imageFile: '//upload.wikimedia.org/wikipedia/en/3/34/Button_hide_comment.png',
            speedTip: 'Comment visible only for editors',
            tagOpen: '<!-- ',
            tagClose: ' -->',
            sampleText: 'Insert comment here',
            imageId: 'button-comment'
        } );
    } );
} );