How can a TinyMCE modal return formatted/visual text?

I am trying to customize the behavior of an editor button in a plugin. I basically want to achieve similar behavior to the “insert code sample” button here.

On click, it opens a modal where the user can input some text. On confirmation, I want to wrap this text into code tags. But I don’t want to take this text as if it comes from a text editor, I want to handle it as visual text. This means, I want to preserve any formatting (whitespaces and linebreaks) but not accept any other tags besides the code tags that I add afterward.

function showDialog() {
    var win = ed.windowManager.open({
            title: "Insert code",
            body: {
                type: 'textbox',
                name: 'code',
                multiline: true,
                minWidth: ed.getParam("code_dialog_width", 600),
                minHeight: ed.getParam("code_dialog_height", Math.min(tinymce.DOM.getViewPort().h - 200, 500)),
                spellcheck: false,
                style: 'direction: ltr; text-align: left'
            },
            onSubmit: function(e) {
                ed.focus();

                ed.undoManager.transact(function() {
                    ed.insertContent('<code>' + e.data.code + '</code>');
                });

                ed.selection.setCursorLocation();
                ed.nodeChanged();
            }
        });
    }