To create your own link, 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. Please keep in mind that this phone number must have an active account on WhatsApp.
After creating your own link, you can style it as a button and then use it on your website.
Find more information on how to leverage your WhatsApp presence in this article.