You can read an article or watch the following video:-
Step 1: Open a messenger bot menu and click the Action button in the Bot settings option.
Step 2: Click on Get Started option.
Step 3: Click on the Enable button to view a message.
Step 4: Enter your Message for the customer. then save it
if you want to send an auto Reply, then Click on Edit get Started reply
Step 5: Select the type of reply like text, with image, with the link, etc.
Enter your message and click on the update button.
Great! Your Bot is now ready to welcome your customer.