Use Copyboard.js to programmatically set a value that will be placed into the clipboard when the user hits Ctrl + C (or Command + C).
http://flackend.github.io/Copyboard.js/
Trello is the inspiration for this. View their implementation on StackOverflow: http://stackoverflow.com/questions/17527870/how-does-trello-access-the-users-clipboard/17528590#17528590.
Include the javascript:
<script src="copyboard.js"></script>
Set the value:
Copyboard.set("Hello, World!");
Now, when the user hits Ctrl + C, "Hello, World" will be placed in their clipboard.