How to add a 2nd light to a two way switch

OK, I have never touched a three way switch before but I want to add a lighted Marquee in my stairwell that leads to the theater. I have a 2' x 4' fluorescent light fixture that I want to add to the switch so when you turn on the light to go down the stairs the marquee comes on and vice versa.

I can access the light switch at the bottom of the stairs and want to tie the black, white and ground wires for the light ficture into the switch but not sure where to put the black? I figure I can tie the ground and white in with the other ones in the switch but not sure where the black goes as there are two black leads coming off the switch. One with a gold screw and one with a black screw.

Any help?

Thanks for your response. I can only get wiring from the new fixture to one of the switches. So I need to run wiring from the new fixture to BOTH switches for it to work?

Basically, I want to add a light fixture at one of the three way switches. The existing light fixture is recessed cans and I cannot get to their wiring without tearing out some drywall so I can only access the wiring at the switch From what I am reading online it says you cannot add a light fixture at the 3 way switch - only by piggy backing on the existing light fixture. Anyone know a way around this?