To get a slam knockout, which also awards you with a trophy/achievement you must attack your opponents head until it’s accumulated enough damage and lost stamina, slam knockouts without damage are almost impossible to do. Once the other fighter is damaged and lost enough stamina, you must then clinch your opponent, and do the slam takedown. A normal takedown or throw will not cause a knockout, it must be the slam takedown.
You should then get the “Throw It On The Ground” achievement/trophy.