Simulating mouse click on Direct Input

Hello,

I'm having a problem with sending a mouse click to a D3D9 window which uses DirectInput for mouse and keyboard access.

I've tried:
Postmessage
Sendmessage
SendInput

Now I've read that SendInput will not work unless you provide a scan code, but I have no idea how to send mouse input using that (only keyboard input).

Any help will be appreciated.

Thanks in advance