Mesej Je for WhatsApp is an app to send WhatsApp messages to other app users without needing to add them to your contacts list. How is this useful? Well, simple: you can close Wallapop deals, get in contact with BlaBlaCar users, talk to a classmate or coworker, etc. Basically, you can talk to anyone via WhatsApp even if you don't want to add them to your contact list.
Using Mesej Je for WhatsApp is really simple: just enter the telephone number of the person you want to talk to and the app will redirect you to WhatsApp, where you can talk to the person as usual. It's really that easy.