Often I'd found useful to use the "Screen Grab TOP" to capture an external application even id it's in background.
So I made some research and I resolved like this:
Download NDICapture.exe from here: https://github.com/idkosilov/NDICapture/releases and put it in the same folder of "grabWindow.toe"
Put inside the "text1" DAT the name (or part of) the window you want to capture.
ATTENTION: if you put just a part of the name consider that it will capture the last window with that name, so be sure to put in a unique string
Also, the window must be visible (not minimized at first run) and the initial size will be the one used for all the capture time
Put inside the "text2" the name of the NDI channel you want to use
Set the "Source name" in "ndiin1"