ThingCreator.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class ThingCreator : MonoBehaviour {
    public GameObject prototype;
    public Transform positionIndicator;

    void Update () {
        if (Input.GetKeyDown(KeyCode.Space)) {
            CreateThing(positionIndicator.position);
        }
    }

    public void CreateThing (Vector3 pos) {
        GameObject created = Instantiate(prototype);
        created.transform.position = pos;
        created.transform.parent = transform.parent;
    }
}

ThingCreatorEditor.cs

using UnityEngine;
using UnityEditor;
using UnityEditor.SceneManagement;

[CustomEditor(typeof(ThingCreator))]
public class ThingCreatorEditor : Editor {
    ThingCreator Target { get => (ThingCreator) target; }

    // https://docs.unity3d.com/ScriptReference/EditorGUILayout.html
    // https://docs.unity3d.com/ScriptReference/GUILayout.html
    public override void OnInspectorGUI () {
        DrawDefaultInspector();
        if (GUILayout.Button("Create!")) {
            Target.CreateThing(Target.positionIndicator.position);
        }

        if (GUI.changed && !Application.isPlaying) {
            EditorUtility.SetDirty(Target);
            EditorSceneManager.MarkSceneDirty(Target.gameObject.scene);
        }
    }

    // https://docs.unity3d.com/ScriptReference/Gizmos.html
    // https://docs.unity3d.com/ScriptReference/Handles.html
    // https://docs.unity3d.com/ScriptReference/IMGUI.Controls.BoxBoundsHandle.html
    public static void DrawGizmos (ThingCreator customTarget) {
        Handles.matrix = customTarget.transform.localToWorldMatrix;
    }

    void OnSceneGUI () {
        DrawGizmos(Target);
        RaycastHit hitInfo;
        Ray worldRay = HandleUtility.GUIPointToWorldRay(Event.current.mousePosition);
        if (Physics.Raycast(worldRay, out hitInfo)) {
            Debug.Log(hitInfo.point);
            if (Event.current.type == EventType.MouseDown) {
                // GUIUtility.hotControl = GUIUtility.GetControlID(FocusType.Passive);
                // Event.current.Use();
                Target.CreateThing(hitInfo.point);
            }
        }

        Handles.color = new Color(1,0,0, 1);
        Handles.DrawSolidDisc(Target.transform.position, Vector3.up, 0.5f);
        if (Handles.Button(Target.transform.position, Quaternion.Euler(90, 0,0),
                           0.5f, 0.5f, Handles.CircleHandleCap)) {
            Debug.Log("!!!!??");
        }

        if (GUI.changed && !Application.isPlaying) {
            EditorUtility.SetDirty(Target);
            EditorSceneManager.MarkSceneDirty(Target.gameObject.scene);
        }
    }
}