Go in the options and choose "Create & Edit Player". Select Shohei Ohtani, press X and select "Edit Player". Go in the attributes, choose "Batting & Fielding" and rise up the contact, power and plate vision attributes up to 99.
Now save your roster, and go to the Play Now menu. Normally, Ohtani is with the Salt Lake Bees (Angels' AAA affiliate), select this team and another one to compete with (you can choose a AA team to be easier) and hit a home run. Be sure to change the gameplay batting difficulty to All-Star.