Pop up a editor window with int input field. Example: Create a menu item on editor, when user select that option, pop up a window with int field. Unlock level based on user's input.

EditorWindowIntField.cs

#if UNITY_EDITOR
using UnityEngine;
using UnityEditor;

class EditorWindowIntField : EditorWindow {

	private static int input = 1;
	private static string _title = string.Empty;
	private static string _btnName = string.Empty;
	private static System.Action<int> _onBtnClick;

    // To create multiple int fileds.
	private static int count = 1;
	private static int[] inputs;
	private static System.Action<int[]> _onBtnClickMultiInputs;

	public static void Create(string title, string btnName, System.Action<int> onBtnClick) {
		_title = title;
		_onBtnClick = onBtnClick;
		_btnName = btnName;
		count = 1;

		Init(title);
	}

    // To create multiple int fileds.
	public static void CreateMultiple(string title, string btnName, System.Action<int[]> onBtnClick, int inputNum = 1) {
		_title = title;
		_onBtnClickMultiInputs = onBtnClick;
		_btnName = btnName;
		inputs = new int[count];
		count = inputNum;

        // Create window after all setting done.
		Init(title);
	}

	private static void Init(string title) {
		var window = (EditorWindowIntField)EditorWindow.GetWindow(typeof(EditorWindowIntField), true, title, true);
		window.Show();
	}

	private void OnGUI() {
		if (count == 1) {
			input = EditorGUILayout.IntField ("Number:", input);
		}
		else {
			for (int i = 0; i < count; i++) {
				inputs[i] = EditorGUILayout.IntField ("Number:", inputs[i]);
			}
		}

		if(GUILayout.Button(_btnName))
		{
			if(_onBtnClick != null && count ==1)
				_onBtnClick (input);
			else if(_onBtnClickMultiInputs != null)
				_onBtnClickMultiInputs (inputs);
			this.Close();
		}
	}
}
#endif

GameDebugger.cs

public static class GameDebugger
{
    [MenuItem ("Tools/Game Debug/Unlock Level by number", false, 1)]
    public static void UnlockLevelByNumber ()
    {
        EditorWindowIntField.CreateNew("Unlock level", "Unlock", input => {
            GameManager.maxLevel = input;
        });
    }
    
	[MenuItem ("Tools/Game Debug/Mission/TestMissions", false, 1)]
	public static void SetTestMissions ()
	{
		EditorWindowIntField.CreateMultiple("Unlock level", "Unlock", (inputs) => {
			MissionManager.SetTestMission (inputs);
		}, 3);
	}

    
}