It's honestly pretty easy to do it by hand. You just push the FD all the way onto the big ring, and even though you bolt down the cable in that position, there's just enough slack for the fd to naturally fall into the small ring once you're done. I suspect if you wedge a tool in there to hold it in place, you'll end up overtightening the cable so that it's slow to shift to the small ring.

Again, I feel that this should be the recommended, standard method for modern Shimano FDs. It's wayyyy better than the typical described method, which is to do it on the small ring, and then to hope that your fine adjuster can be turned all the way out. (Mine never reached quite enough far enough out.) With the hand-push method, you can actually leave your adjuster in the 'middle' position since the cable will be plenty tight so that you can go up or down on the length to get it perfect.