Basic example

jsPanel.create({
    content:     '<textarea id="simplemde"></textarea>',
    contentSize: '550 350',
    headerTitle: 'jsPanel with SimpleMDE',
    theme:       'primary',
    callback: function (panel) {
        var simplemde = new SimpleMDE({
            element: document.getElementById('simplemde')
        });
        $('.CodeMirror-wrap', panel.content).css({height: 'calc(100% - 83px)'});
        $('.CodeMirror', panel.content).css({minHeight: 0});
        $('#btn-1').attr('disabled', 'disabled');
        panel.header.style.backgroundImage = 'linear-gradient(120deg,#155799,#159957)';
    },
    onclosed: function () {
        $('#btn-1').attr('disabled', false);
    }
});