Do you plan on making the headboard from soled lumber? If so, you need to do some engineering.
You are creating a 14" long cross grain joint. The headboard will want to expand and contract with changes in relative humidity. Unless you design a way for the headboard to move, you will end up with splitting or other damage as the wood moves.
The way we dealt with this in my shop was to make two M&T joints in each side of the headboard and post. The top M&T would be constructed normally with a 1 1/2" to 2" tenon mortise. It would be glued and pinned with a 3/8" dowel inserted from the back so that the dowel was not visible from the front. For the bottom of the headboard we would again make a 1 1/2" to 2" tenon just like the upper joint. But, this joint will not be glued. Instead we would make a matching slot for the pin allowing the headboard to move up and down as the wood expanded and contracted. (Think of the joint used on a breadboard end.)
With a 14" wide joint you might want to consider three M&T joints per side.
I have seen other solutions to the problem and you may be able to come up with something different. Just be sure you keep wood movement in mind.