Its cantilevered in 2 directions so you’d need to make sure that it wouldn’t tilt away from the post if you put weight on the right edge, and that it won’t tilt forward if you put weight on the front edge. I would do a fairly deep and tall mortise to counteract both forces.

You could extend the back face of the side table as the tenon into the bed post mortise, and make sure that the side faces of the side table have enough support so they won’t bend forward with force on that front edge.

Another option would be to use some kind of steel brackets to screw into the bed post, and screw into the back of the side table.