EventSystem: prefabs that aren't in the scene don't accept GameObject

I read about UnityEvent and am trying to implement it. I have a "GameController.cs" script, attached to a GameObject "ScriptsHolder", and a "target.cs" script attached to a Prefab GameObject called "target". "target" is not in the scene on startup: I initiate multiple targets at a certain point, which are renamed to "target0", "target1", ...

When a target receives damage, I want it to send its name to GameController.cs, which has a void that destroys that object.

Here's what I did:

target.cs:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Events;
 
 public class target : MonoBehaviour {
 
      public UnityEvent damageEvent;
 
     void Damage(float damage)
     {
         damageEvent.Invoke();
     }        
 }

Next, I click on my target Prefab and in the inspector, I try to add "ScriptsHolder" to the Damage Event. Here's my problem: I can't drop the ScriptsHolder in the Damage Event. Did some testing, and the reason for this appears to be the fact that "target" is not yet in my scene.

So I read about adding a listener instead, but I can't figure out how to do this. Here what I have in "GameController.cs":

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Events;
 public class GameController : MonoBehaviour {
 UnityEvent damageEvent;

 void Awake()
 {
     damageEvent.AddListener(targetReceivedDamage);
 }
 void targetReceivedDamage()
 { // do some stuff }
 }

But that doesn't work. What am I doing wrong? Or how can I add the ScriptsHolder to the Damage Event even though "target" isn't in the scene?

Any help is much appreciated!