In my Dell laptop, Fn + F6 is Video Output button. (i.e LCD Off and D-sub Monitor On)

How can I write some programs to do this function?

When I hook INT 9 in real mode, this fn+f6 doesn't do INT 9 (IRQ1).

fonolite,

Fn+key combinations are (most probably) handled by BIOS code running in SMM. While it may be possible to examine SMRAM contents' disassembly and find appropriate instructions' sequence, I suppose you can find datasheet for your videocontroller that contains the information that code is based upon.

It may well also be completely impossible. I've seen some combinations of Fn + key returning scan codes that are impossible to differentiate from single keys on a desktop keyboard. I reckon hardware functions could be present, that is, in this case, the key combination could directly trigger a hardware switch instead of going through the BIOS.
If normally, when you press a Fn key combination, you see a pop up on the screen with some graphics, it could be software based, but "silent" keys may be just hard wired.
Of course, I'm just guessing.

"Hard wired" keys is very unlikely. Too much effort to make such a thing. The chips used in keyboards are extremely cheap and generic and there is no need for a manufacturer to use something custom. So based upon that all keys are handled by software. Be that the OS kernel or SMM it will all be software processed.