The 5200 doesn't have commander mode, so you can't control the SB-910 through iTTL.
However, you can put the SB-910 into optical slave mode so that it fires whenever it sees a flash go off. So what you can do with the d5200 is put it on the lowest flash power setting so that the on-camera flash contributes very little to the exposure, but the SB-910 would still see the flash and then go off. You would need to manually control the power of the SB-910 in this case though.
Just fyi, TTL means through-the-lens metering. So basically the camera controls the exposure of the flash to tell it how hard to light a subject. Using optical slave mode instead of TTL to the SB-910 means the camera can't communicate with the SB-910, only trigger it, and thus it cannot do TTL metering.
You can buy pocketwizards that support TTL for Nikon, but most people end up just using them for the wireless non-TTL communication since that trigger flash from your D5200 ends up always adding a little to the exposure and doesn't always work in larger rooms, outdoors, or weird angles.
Edit: I'm not 100% sure the D5200 can do TTL metering even with pocketwizards you might want to look into that.