Attach the ObjectManipulator MRTK component in runtime to instantiated GameObject?

I am building an XR experience for Mobile platforms. For this, I import a glTF model to my scene through a GameObject when it loads and instantiate it in the scene. This part is ok.

What I want to achieve is that after the GameObject containing the model is instantiated, I want to attach an MRTK script to it, but when I try to use the GameObject.AddComponent<> method I can't see the name of the script.

Is there any specific way to reference the script if it is within my Assets?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.SceneManagement;
 using Microsoft.MixedReality.Toolkit.XRSDK.Input;
 
 public class ObjectManager : MonoBehaviour
 {
     private GameObject placedModel;
 
     //This runs first
     void OnEnable()
     {
         Debug.Log("OnEnable called");
         SceneManager.sceneLoaded += OnSceneLoaded;
     }
 
     //this runs second
     async void OnSceneLoaded(Scene scene, LoadSceneMode mode)
     {
         Debug.Log("OnSceneLoaded: " + scene.name);
 
         var gltf = new GLTFast.GltfImport();
         var success = await gltf.Load("https://raw.githubusercontent.com/KhronosGroup/glTF-Sample-Models/master/2.0/Duck/glTF/Duck.gltf");
 
         if (success)
         {
             // Instantiate the glTF's main scene
             gltf.InstantiateMainScene(new GameObject("PlacedModel").transform);
 
         }
         else
         {
             Debug.LogError("Loading glTF failed!");
         }
     }
 
     //this runs third
     private void Start()
     {
         placedModel = GameObject.Find("PlaceModel");
         placedModel.AddComponent<ScriptName>;
     }
 
 }