WhatsApp has been working on message reactions for some time now. Some leaks have shown how the feature will look like on your phone as well as on web version. At the moment the number of reaction emoji is limited but that is going to change soon.
According to some reports, WhatsApp beta for Android 2.22.10.9 provides us a glimpse at how you can add emoji reactions other then five standard ones. When you will tap a message to send a reaction you will see a plus symbol at the right of the reaction carousel. Upon tapping it you will be directed to familiar WhatsApp emoji picker where you can select any emoji of your choice. The new reaction picker is rolling out to the ones using the latest beta version but there is a server-side switch involved so it is not available for everybody at the moment. Earlier it was reported that message reactions went live for some people after updating the beta version 2.22.8.3 but they disabled it soon after.
Emoji reactions seems to be very limited test that has reached to few users. If you are looking for trying your luck you can download the latest beta version of the app. If you can also try your luck by downloading the latest beta version of the app.