What you should try is to remove onClick and trigger postback manually with __doPostback. use this link as a guide:

__doPostBack function - CodeProject

function startTimer(e) {
    e.preventDefault();
    startMinutes = 1; 
    time = startMinutes * 60;
    btnSubmit.prop('disabled', true);
    btnSubmit.val('Resend');
    timerElement.style.display = 'block';
    if (!interval) {
        interval = setInterval(updateTimerCountdown, 1000);
    }
    __doPostack('btnSubmit', '');
    return true; 
}