You will need to get the send button code for your site from this address:- https://developers.facebook.com/docs/plugins/send-button
And the code is below
<div id="fb-root"></div>
<p>{literal}</p>
<script>// <![CDATA[
(function(d, s, id) {
var js, fjs = d.getElementsByTagName(s)[0];
if (d.getElementById(id)) return;
js = d.createElement(s); js.id = id;
js.src = "//connect.facebook.net/en_US/sdk.js#xfbml=1&version=v2.5&appId=205496139493191";
fjs.parentNode.insertBefore(js, fjs);
}(document, 'script', 'facebook-jssdk'));
// ]]></script>
<p>{/literal}</p>
<div style="-moz-border-radius: 15px; background: -webkit-linear-gradient(top, #FFFFFF, #E0E0E0 ); border-radius: 15px; border: 1px solid #000000; padding: 10px;"><table><tbody><tr><td align="left"><div class="fb-send" data-href="https://americymru.net/user/signup"></div></td><td>Invite your Facebook friends to join AmeriCymru: </td></tr></tbody></table></div>--
Ceri Shaw - AmeriCymru
updated by @adolygwr: 05/01/16 10:03:52PM
