Setting a Greeting and Get Started command
Once you have successfully integrated your bot with Facebook Messenger there are optional Messenger features that you can use to customize the experience.
Starting a conversation with your bot on Facebook Messenger
Greeting your users
The greeting field allows you to specify a message that will be displayed on the welcome screen of your page on Messenger. Note that the welcome screen is only displayed the first time a user interacts with your page.
See Facebook's documentation here
Filling in the Get Started field will enable the Get Started button on your page on Messenger. Like the greeting, this is displayed on the welcome page the first time the user interacts with your bot.
Whatever text you put into this field will be sent to the bot when the user presses the Get Started button. Your bot should reply to this with a menu or some information to help the user understand how to use the bot.
See Facebook's documentation here.