<a href="intent://send/+XXXXXXXXXXX#Intent;scheme=smsto;package=com.whatsapp;action=android.intent.action.SENDTO;end">Open WhatsApp chat window</a><a href="intent://send/+XXXXXXXXXXX#Intent;scheme=smsto;package=com.whatsapp;action=android.intent.action.SENDTO;end">Open WhatsApp chat window</a>