In jwplayer 6 it simply can't be done. Downgrade to jw5 and it will be do-able.
I think the guys at jwplayer are plain stupid. This, and other 'features' in the new jw6 such as the unchangeble vertical volume slider, etc. makes using this player almost impossible for the majority of us. Removing very old and used features from a product is like commiting suicide for a company. It didn't happen YET because there are not many alternatives for jw. Eventually there will be, so good luck with that amazingly clever business strategy, jw !
P.s. Beaware that the geniuses at jwplayer are no longer supporting jw5, so you will not find any documentation on that topic. Also the source code for older versions is also no longer available, giving the fact that they switched from their developers platform to github, and they "forgot" to migrate the old player files.
Good Luck