The former Notre Dame standout started Heyward out with a two-seamer and a splitter which were both low in the zone. The third pitch ended up right in Heyward's hot spot, as you can see from the heat map above. It was a 95 MPH two-seamer that Heyward hit out over the wall in right field at Wrigley.
Samardzija tried to keep the ball low on Heyward all day, which is odd considering Heyward generates most of his power on balls low in the zone. He was likely hoping to get Heyward to chase a few pitches down and out of the zone, and missed his spot on the HR pitch. From the video link above, you can see that Soto was actually setting him up outside.
Given the kind of velocity Samardzija can get behind his fastball, he might do better trying to come up and in on Heyward in future meetings.