WhatsApp on Monday announced that it is now allowing users to reserve their usernames on the instant messaging platform, in preparation for the upcoming feature, which will allow you to search for other known and unknown WhatsApp users, bringing the instant messaging platform closer to how other social media platforms, like Instagram, function. Confirmed to roll out soon, the feature will eliminate the need for you to share phone numbers with other users to allow them to contact you via WhatsApp. You will be able to simply share your usernames with others, and they will then be able to search for you within the app. However, if you do not want everyone to contact you, you can choose to set a unique username key. Others will not be able to contact you if they do not have this key.
How to Reserve Your Username on WhatsApp: A Step-By-Step Guide

