Mobile website "WhatsApp" button to send message to a specific number

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?

Format to send a WhatsApp message to a specific number (updated Nov 2018)

<a href="https://wa.me/whatsappphonenumber/?text=urlencodedtext"></a>

where

whatsappphonenumber is a full phone number in international format

urlencodedtext is the URL-encoded pre-filled message.

Example:

1. Create a link with a pre-filled message that will automatically appear in the text field of a chat, to be sent to a specific number

   Send I am interested in your car for sale to +001-(555)1234567

   https://wa.me/15551234567?text=I%20am%20interested%20in%20your%20car%20for%20sale

   Note :

   Use: https://wa.me/15551234567

   Don't use: https://wa.me/+001-(555)1234567

2. Create a link with just a pre-filled message that will automatically appear in the text field of a chat, number will be chosen by the user

   Send I am enquiring about the apartment listing

   https://wa.me/?text=I%20am%20enquiring%20about%20the%20apartment%20listing

   After clicking on the link, user will be shown a list of contacts they can send the pre-filled message to.

For more information, see https://www.whatsapp.com/faq/en/general/26000030

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P.S : Older format (before updation) for reference

<a href="https://api.whatsapp.com/send?phone=whatsappphonenumber&text=urlencodedtext"></a>

WhatsApp is now providing a much simpler API https://wa.me/ This isn't introducing any new features, just a simpler way to execute things. There's no need to check for user agent while implementing this API as it will also work with native apps as well as the web interface of whatsapp (web.whatsapp.com) on desktop.

This can be used in multiple use cases

• A Click to chat button : Use https://wa.me/whatsappphonenumber to open a chat dialog with the specified whatsapp user. Please note that the whatsappphonenumber should be a valid whatsapp number in international format without leading zeros, '+', '-' and spaces. e.g. 15551234567

  <a href="https://wa.me/15551234567">Whatsapp Me</a>

• A Share this on whatsapp button : Use https://wa.me/?text=urlencodedtext to open a whatsapp contact selection dialog with a preset text. e.g.

  <a href="https://wa.me/?text=I%20found%20a%20great%20website.%20Check%20out%20this%20link%20https%3A%2F%2Fwww.example.com%2F">Share on WhatsApp</a>

• A Contact me button with prefilled text : A combination of the above two, Might be useful if you want to get a prefilled custom message from users landing on a particular page. Use format https://wa.me/whatsappphonenumber/?text=urlencodedtext

  <a href="https://wa.me/15551234567?text=I%20am%20interested%20in%20your%20services.%20How%20to%20get%20started%3F">I am interested</a>

For official documentation visit https://faq.whatsapp.com/en/general/26000030