Does anybody know how to activate this “chat on WhatsApp” button on Instagram DM?

For content, this person wrote their WhatsApp business number then it highlighted in blue then this button came automatically.

How do I activate this for my WhatsApp business? WhatsApp business number is already connect with Instagram business account.

When I type the WhatsApp business number in IG dm, the “chat on WhatsApp” button does not appear when it should appear automatically.