How to set camera to slow sync flash?

I want to set my camera to a slow sync flash setting (if possible with red-eye reduction). I'm taking a portrait in the night, that i want the scenery behind to show as well.

i have nikon d5100, 18-55mm & 55-200mm lenses, built in flash.
the max sync speed for my nikon is 1/200 sec.

the problem is that, when going to the flash setting menu, only 3 options for flash are coming regardless of the shutter speed selected: auto, auto +red-eye reduction, off. The camera is set to manual settings, so i suppose i should change my flash.

any insights?

Look on page 51 & 52 of your user manual. As you will see, you have to be in either the Shutter Priority or Manual Mode to use rear curtain sync.

When in the Program and Aperture Priority modes, you can shoot rear curtain + slow sync.

There's more on page 204 that discusses the use of various Nikon flash units