erpnext/templates/includes/footer_extension.html

<div class="container">
	<div class="row">
		<div class="col-sm-6 col-sm-offset-3" style="margin-top: 7px;">
			<div class="input-group">
				<input class="form-control" type="text" id="footer-subscribe-email"
					placeholder="{{ _('Your email address') }}...">
				<span class="input-group-btn">
					<button class="btn btn-default" type="button"
						id="footer-subscribe-button">{{ _("Stay Updated") }}</button>
				</span>
			</div>
		</div>
	</div>
</div>
<script>
	$("#footer-subscribe-button").click(function() {

		if($("#footer-subscribe-email").val()) {
			$("#footer-subscribe-email").attr('disabled', true);
			$("#footer-subscribe-button").html("Sending...")
				.attr("disabled", true);
			erpnext.send_message({
				subject:"Subscribe me",
				sender: $("#footer-subscribe-email").val(),
				message: "Subscribe to newsletter (via website footer).",
				callback: function(r) {
					if(!r.exc) {
						$("#footer-subscribe-button").html("Thank You :)")
							.addClass("btn-success").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 email address"))
	});
</script>