Plastering a gap around a window
I don't know where in the world you are, but in the U.S. no one fills that gap with plaster. We install "casing", or trim frames, around the window. Casing laps onto both the window jamb and the wall. 



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If you were to fill with plaster you'd want to reinforce the joint with fiberglass tape or similar. The seams between the new plaster, the wall, and the window are sure to crack.