Rashi on Exodus 10:1/רש"י על שמות י׳:א׳

1ויאמר ה' אל משה בא אל פרעה AND THE LORD SAID UNTO MOSES GO IN UNTO PHARAOH and warn him.

2שתי means my setting (an infinitive of שית), so that the phrase, which literally means “for the sake of my setting these my signs”, denotes “that I may set”.