title: Mobile website "WhatsApp" button to send message to a specific number
body: <p>A mobile website can be customized to allow users to share a pre-filled message in WhatsApp to a manually chosen contact. As given <a href="https://faq.whatsapp.com/en/android/28000012" rel="noreferrer">here</a> it is done using Custom URL Scheme. An example:</p>
  
  <pre><code>&lt;a href="whatsapp://send?text=Hello%20World!"&gt;Hello, world!&lt;/a&gt;
  </code></pre>
  
  <p>To call a particular number we use:</p>
  
  <pre><code>&lt;a href="tel:0123456789"&gt;Call&lt;/a&gt;
  </code></pre>
  
  <p>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?</p>