Check different ways
API way:
Wa.me
Or:
Whatsapp protocol
<a href="whatsapp://send?phone=0031103073955&text=I%20would%20like%20to">Whatsapp protocol</a>
This doesn't show up as a clickable link due to github (starts with whatsapp://). Worse, it does not accept a phone parameter.
This is recommended on https://faq.whatsapp.com/android/im-an-android-developer-how-can-i-integrate-whatsapp-with-my-app but it doesn't work.
Or for Android users (start with intent://):
<a href="intent://send/0031103073955#Intent;scheme=smsto;package=com.whatsapp.com;action=android.intent.action.SENDTO;end">Whatsapp intent</a>
Whatsapp intent
Try for example on https://www.w3schools.com/html/tryit.asp?filename=tryhtml_intro
Nothing of the above works. However! If you first go to Firefox on your mobile phone, Settings, Advanced, and Open links in apps. See https://support.mozilla.org/en-US/kb/set-firefox-android-open-links-native-apps. Then it works! Use a plus sign rather than 00 for the country code: