brotherton-erpnext/erpnext/templates/includes/footer/footer_extension.html

{% if not hide_footer_signup %}
<div class="input-group">
	<input type="text" class="form-control"
		id="footer-subscribe-email"
		placeholder="{{ _('Your email address...') }}"
		aria-label="{{ _('Your email address...') }}"
		aria-describedby="footer-subscribe-button">
	<div class="input-group-append">
		<button class="btn btn-sm btn-default"
			type="button" id="footer-subscribe-button">{{ _("Get Updates") }}</button>
	</div>
</div>

<script>
frappe.ready(function() {
	$("#footer-subscribe-button").click(function() {

		if($("#footer-subscribe-email").val() && validate_email($("#footer-subscribe-email").val())) {
			$("#footer-subscribe-email").attr('disabled', true);
			$("#footer-subscribe-button").html("Sending...")
				.attr("disabled", true);
			erpnext.subscribe_to_newsletter({
				email: $("#footer-subscribe-email").val(),
				callback: function(r) {
					if(!r.exc) {
						$("#footer-subscribe-button").html(__("Added"))
							.attr("disabled", true);
					} else {
						$("#footer-subscribe-button").html(__("Error: Not a valid id?"))
							.addClass("btn-danger").attr("disabled", false);
						$("#footer-subscribe-email").val("").attr('disabled', false);
					}
				}
			});
		}
		else
			frappe.msgprint(frappe._("Please enter valid email address"))
	});
});
</script>

{% endif %}