Check this link out
Launching Your iPhone App Via Custom URL Scheme
and more on the whats up url scheme document
I did a quick mock up and tried it on my iphone with a link like this from a webpage and it opened the app on my iphone.
<a href="whatsapp://send?text=Hello%2C%20World!">whatsapp</a>
I could not try to send a message as I don't have a current Whatsapp account sorry.
Add user name using abid parameter
let's say your whatsapp username was username then it would be
<a href="whatsapp://send?abid=username&text=Hello%2C%20World!">whatsapp</a>
once again sorry I can't test this. Also I have no idea what would happen if the username is the actual user of the current mobile device. eg. You try to whatsapp yourself.