Read text from others process UI that is not selectable at all

i have a pretty old application that looks like a normal windows forms app. But there is one text field containing text that is even not selectable. I want to read this piece of text in terms of an automated UI test. I already tried to inspect this window with AutoIT v3 Window Info. But apart from the window title, it is not possible for the "Window Info"-Tool to see any of the ui elements inside the window. Is it possible to listen to the message queue of this application with AttachThreadInput and react on WM_SETTEXT messages (store the message content)? Perhaps one of these events contains the wanted text? But how could i achieve this?

Best regards, Teimo