First reported by tecnoblog (translated from Portuguese), Instagram is now showing users an option to share Stories on WhatsApp under the ‘Your Story’ option that appears after you’ve completed recording a story. This feature has been confirmed to be under testing with a select group of users by Instagram, who added:
We are always testing ways to improve the experience on Instagram and make it easier to share any moment with the people who matter to you.
This cross-posting feature has been built using WhatsApp’s publicly available code, which makes it possible to integrate the Facebook-owned messenger into other 3rd party apps. Even Google Assistant makes use of this library to allow users to send messages with voice commands on Android devices.
As seen in the screenshots attached below, you can tap the Your Story button (shown on the left) and then activate WhatsApp sharing. Once you’ve shared your Instagram Stories on WhatsApp, you’ll see an Instagram logo appear on the bottom right of the same.