How do i make a reference to another game manager? Not game object

I made a reference here but the drop down list in script seems to not contain the variables from another script

 [SerializeField] private gm MainPlatform;  public Transform SidePlatform; private Vector3 NextSpawnSidePlatform; void Start() {     MainPlatform = GameObject.FindWithTag("gameManager").GetComponent<gm>();     NextSpawnSidePlatform.x = 30.5f;     StartCoroutine(SpawnSidePlatform()); }