How to enable contiuous auto focusing during video recording on nikon d5200?

It has it but i'm not sure how to enable it.

Read the manual:
http://www.nikonusa.com/...(11)01.pdf

It depends what mode the camera is in.
In Live view mode, start reading on page 94… All the way through page 100.
Page 112. "Autofocus is not available during movie recording."

Remember, a dSLR is designed for capturing still images. It can do that well. Video and audio capture is a secondary convenience feature. This does not mean your camera can't capture good video, it can, but it can't be treated like a camcorder because it is not a camcorder. Watch out for overheating and file size limitations.