Open Whatsapp when you click on the icon

Question

Does anyone know how to make that by clicking on the Whatsapp icon on a website by smartphone, some function open the application and already start a conversation at the number set in link of the site?

Imagine an icon that says "attendance by Whatsapp [phone]" and clicking there already start a conversation on Whatsapp direct at this number xxxx-xxxx? Any idea?

Answer 1

<a href="intent://send/NUMERO_TELEFONE#Intent;scheme=smsto;package=com.whatsapp;action=android.intent.action.SENDTO;end">
<img src="img/whatspng.png"></a>

Answer 2

Place an icon and click this link with the desired number.

https://api.whatsapp.com/send?phone=[phone]