Here is some code I used to first record the mouse position (relative mouse position did not work since the window was tiled and the click position could be changed by moving the tiles). Immediately after that I click the mouse at the recorded position (and redo this later on in the code as well).
SplashTextOn, 200, 100, Script Preparations, Please Click on the Person `"Search Term`" link (1) in SAP. ; Show new instructions to the user
WinMove, Script Preparations,, (A_ScreenWidth/2)-150, (A_ScreenHeight/2)-200 ; Move the text instructions window with the name "Script Preparations" 150 pixels right of the center of the screen and 200 pixels up SoundBeep 600, 300 ; Wake up user
; From here on the left mouse button will temporarily be disabled
Hotkey, LButton, on ; Turn Left Mouse Button OFF, to capture the Mouse click
KeyWait, LButton, D ; Wait for LeftMouseButton click Down
MouseGetPos, xposS ,yposS ; Store the position where the mouse was clicked (Search Box)
MouseClick, left, %xposS% ,%yposS% ; Perform the mouse click on the captured mouse location
Hope this helps.
In your situation, I assume that you only need to determine the mouse position with Window Spy.