A mobile website can be customized to allow users to share a pre-filled message in WhatsApp to a manually chosen contact. As given here it is done using Custom URL Scheme. An example:
<a href="whatsapp://send?text=Hello%20World!">Hello, world!</a>
To call a particular number we use:
<a href="tel:0123456789">Call</a>
Similarly, can we send a WhatsApp message to a specific number (or at least open the chat) without user choosing the phone number manually rather it will be one of the predefined parameters/attribute values?