It takes more than just sending keystrokes via the PS2 protocol.

When the PC is booting, the keyboard must send and receive several commands to tell the PC what it is, what mode it's using, etc.

Go to the link Sayzer gave in post #4, then click on "PS/2 Keyboard Interface".
Look at the Command Set/Initialization sections at the bottom.

It's possible to place the device in parallel with an existing keyboard, so the keyboard does all the initializing.
But if you don't want the keyboard, you'll need to do it all yourself.

Not as easy as just toggling a couple pins with the right timing.
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