Fill a saucepan with water and bring it to a boil. Add the shrimp and cook for 1 minute, or until they turn pink and are cooked through. Drain the shrimp and set them aside.
Next, prepare the dressing by combining minced garlic, ketchup, sugar, and olive oil in a small bowl. Whisk well until the sugar is completely dissolved, and set aside.
In a salad bowl, combine the shrimp, tomato, cucumber, and dressing. Toss to mix well. Transfer the salad to a plate and serve immediately, garnished with scallions on top.