Download the current release or clone the repo and upload dist/whatsapp-button.js somewhere on your webserver.
Use this code right before the </body>, replace http://your-domain.tld/path/whatsapp-button.js with the url to the just uploaded file.
<script type="text/javascript"> if(typeof wabtn4fg==="undefined") {wabtn4fg=1;h=document.head||document.getElementsByTagName("head")[0],s=document.createElement("script");s.type="text/javascript";s.src="http://your-domain.tld/path/whatsapp-button.js";h.appendChild(s)}</script>
Place this code somewhere on your website to display the button:
<a href="whatsapp://send" data-text="Take a look at this awesome website:" data-href="" class="wa_btn wa_btn_s" style="display:none">Share</a>
If you want to customize the size, style, url or the sharing text, feel free to visit whatsapp-sharing.com for a code generator.