Get most (or all) messages of a window

I created a AutoIt script, which open 2 programs, say A and B, and use SetParent to embed B into A.

I have to deal with the size of a child window of A ( say C ) and B.

But I don't know how to get WM_SIZE message of these windows in my AutoIt script, since these windows are not created by this script.

Is it possible to get winapi messages, in AutoIt script,  of "external" windows??