How to add trigger system in my enemy spawner script/

 public class spawner : MonoBehaviour
 {
     private int waveNumaber = 0;
     public int enemySpawnAmount = 0;
     public int enemykilled = 0;
 
     public GameObject[] spawners;
     public GameObject enemy;
 
     private void Start()
     {
         spawners = new GameObject[2];
 
         for (int i =0; i < spawners.Length; i++)
         {
             spawners[i] = transform.GetChild(i).gameObject;
         }
 
         Startwave();
     }
 
     private void Update()
     {
 
         if(Input.GetKeyDown(KeyCode.T))
         {
             SpawnEnemy();
         }
         if (enemykilled >= enemySpawnAmount)
         {
             Nextewave();
 
         }
         
     }
     
     private void SpawnEnemy()
     {
         int spawnerID = Random.Range(0, spawners.Length);
         Instantiate(enemy, spawners[spawnerID].transform.position, spawners[spawnerID].transform.rotation);
     }
 
     private void Startwave()
     {
         waveNumaber = 1;
         enemySpawnAmount = 3;
         enemykilled = 0;
 
         for(int i = 0; i < enemySpawnAmount; i++)
         {
             SpawnEnemy();
         }
     }
 
     public void Nextewave()
     {
         waveNumaber++;
         enemySpawnAmount += 2;
         enemykilled = 0;
 
         for (int i = 0; i < enemySpawnAmount; i++)
         {
             SpawnEnemy();
         }
     }
 }