To add the "Greeting Message" for your users, the first step is to go to the settings.
From the settings, go to the Greeting Message
In the message section, add the message you want to show to the users.
Choose the variable you want to address to your users such as; First name, Last name, or Full name
At last, save the Greeting message by simply clicking on the save button
Click the "Test your bot here" link.