Jacob responds with the true prophetic exasperation of those who seek for signs. Jacob declares that he would not request such a thing of God. However, at some point the Lord tells Jacob that he should pronounce the curse. Jacob is the agent of God, and clarifies in his statement that the cursing comes from God, not Jacob.
Fittingly, the cursing comes upon Sherem directly. Jacob focuses the cursing as the precise proof of the argument. If Sherem is cursed, it is therefore proof that "[God] has power, both in heaven and in earth; and also, that Christ shall come."