Postmessage function in QTP for simulating Printscreen key does not work

[steelbird]

I am using the following code to simulate a PrintScreen button press to capture the image of a Internet Explorer window. I have assigned Printscreen as a Hotkey in Greenshot tool to capture the image of the full page in one go without having to scroll.

Code:

Extern.Declare micLong, "PostMessage", "user32.dll", "PostMessageA", micHwnd, micLong, micLong, micLong

Const WM_KEYDOWN = 273
Const VK_SNAPSHOT = 44

iResult = Extern.PostMessage( Hwnd, WM_KEYDOWN, VK_SNAPSHOT, 0)

But somehow the value of iResult is always 1. Hwnd is the actual window handle of the Internet explorer window which is passed to the Postmessage function. The reason why I am using this approach is because the screenshot capturing process needs to be automated completely and should run when the system is locked as well. SendKeys does not work when system is locked and so I had to move to Post Message.

Could someone please help me out with this.