olark.configure('WelcomeAssist.welcome_messages', ['hello world', 'another message!', 'a 3rd message!']);

Notes

Set the welcome messages to send to the visitors when the Greeter function activates. If you have more than 1 message enabled in the call, we will choose a random one to send to the visitor when they meet the greeter criteria.

You can also enable this on the Greeter extension page.