There is a 60-minute interview with Esper that he confirmed this.
I can't copy the link for some reason. Look up David Leavitt on tweeter.

Donald Trump’s former Sec. Def. Mark Esper: “[Trump] was suggesting that...we should bring in the troops and shoot the protesters.”

Q: “The commander-in-chief was suggesting that the U.S. military shoot protesters?”

Esper: “Yes, in the streets of our nation’s capital.”