ExampleClickSystem.cs

using Unity.Entities;

public class ExampleClickSystem : ComponentSystem
{
    protected override void OnUpdate()
    {
        Entities.ForEach((ref PointerEvent pointerEvent) =>
        {
            if (pointerEvent.EventType != PointerEventType.Click)
                return;

            var pointerEntity = pointerEvent.Entity;
            UnityEngine.Debug.Log("CLICKED " + pointerEntity.Index)
        });
    }
}