The headers and beforeSend parameter does not work.

var calendar = new FullCalendar.Calendar(calendarEl, {
    headerToolbar: {
        start: 'title', 
        center: 'dayGridMonth',
        end: 'prev,next'
    },
    eventSources: [{
        url: 'get_data',
        extraParams: params,
        method: 'POST',
        headers: {
            'custom_param': 'val',
        },
    },],
});
calendar.render();

I want to send some custom parameters on request header which is used by server.