Code

<div id="entry-source-codemirror"></div>

Load CodeMirror on the page before Redactor (included on the documentation site).

const app = Redactor('#entry-source-codemirror', {
    codemirror: {
        mode: 'xml',
        lineNumbers: true
    }
});

Open HTML in the toolbar to switch to source mode with CodeMirror.