<input value="Send an Email" class="btn" name="email" onclick="navigateToUrl('/_ui/core/email/author/EmailAuthor?p2_lkid=00QU0000008xZYi&rtype=00Q&retURL=%2F00QU0000008xZYi','RELATED_LIST','email');" title="Send an Email" type="button">

document.getElementsByName("email").click()

document.getElementsByName("email")[0].click()

document.getElementsByName("email")[0].click()