💡 Note: You can only include one action button in your Instagram profile at a time.
Step-by-step instructions:
Go to your Business profile on Instagram (your icon in the bottom right corner). If you're not sure whether or not you have a Business account on Instagram, you can check by following the steps here.
Tap on Edit Profile.
Under Public Business Information, tap on Contact Options.
Tap on WhatsApp Business Phone Number.
Add your number (or select it if it is already linked). Then press Send Code.
You will now find the button under Contact.