I’ve posted client script part above, here is the suitelet:

function onRequest(context) {
        var form = createForm();
        if (context.request.method == 'POST') {
            var form = serverWidget.createForm({
                title: 'POST'
            });
            context.response.writePage({
                pageObject: form
            });  
        } else {
            var form = createForm();
            context.response.writePage({
                pageObject: form
            });            
        }
    }

So I’m starting suitelet with get request from UI, it generates form with the button, I press that button in UI, send the POST to suitelet, but it doesn’t update form (though I can see that post request and response in the chrome dev console)