How do I pre-fill a Tweet Box with text from Javascript?

I have a site that writes poems using Javascript, very similar to this site: http://ift.tt/g87JEJ

I want users to be able to tweet their custom poem.

How can I make it so that the user's poem appears inside the Tweet text box, after they press the "Tweet" button?

I am only using JS and HTML in this project. Thank you.