You can not add a WhatsApp Icon to an Instagram Post.
In Instagram you can add text and hashtags but you can not add a link, an icon or anything else.
Instagram is very protective in terms or community. They do not want to have links to the outside web.
What you can do to have a link to WhatsApp from your Instagram Channel
There is a solution to have a link to WhatsApp and it is in your Channel Home Page.
In your Instagram Home Page you can add a link. You can have a link to your Website or linkedin profile or WhatsApp.
How to create a link to your WhatsApp Channel
You can use our tool that will generate a link to your WhatsApp Channel.
This tool is free and very easy to use.