window top flashing/channel against flat surface
If I understand this correctly, you need a way to connect the flashing to the parging. I would advise studying the way that chimney apron flashing is attached to chimneys. In short, (maybe call a contractor to) cut a 1/2" deep groove (aka kerf or riglet) in the parging to insert the flashing. Use window and door flashing tape (aka double stick) and silcone to adhere the flashing.

If you hare having trouble attaching the flashing to the window or something, then (maybe upload a picture and) describe the problem more specifically.