Currently WhatsApp allows you to create a click to chat link. To create your own click to chat link to use in your signature use https://wa.me/<number> where the <number> is a full phone number in international format. Omit any zeroes, brackets or dashes when adding the phone number in international format. For a detailed explanation on international numbers, read this article. Please keep in mind that this phone number must have an active account on WhatsApp.
Don't use: https://wa.me/+001-(555)1234567
To create your own link with a pre-filled message that will automatically appear in the text field of a chat, use https://wa.me/whatsappphonenumber/?text=urlencodedtext where whatsappphonenumber is a full phone number in international format and URL-encodedtext is the URL-encoded pre-filled message.
To create a link with just a pre-filled message, use https://wa.me/?text=urlencodedtext
After clicking on the link, you will be shown a list of contacts you can send your message to.