Basically, it’s all very dependent on what Front-end/Framework you use to present your bot. @saurabh-m523 's answer is pretty specific to the concrete Web component he sent you.
But in a nutshell, all you have to do is let your framework/front-end send an initiating message to the rasa bot for each new session with the content “/intent_name”, where “intent_name” is the name of the intent that provokes the desired greeting message.