How to create custom inspector from my script (dropdown from list)

Hello people
I have this script to create 2 lists getting the values from a text file.
I use the first list to complete a dropdown options and later I use the dropdown to set the selected value into a 2 strings.

The idea always was make this work in the inspector, but I really dont know how to make it.

I added: [ExecuteInEditMode]
with this, the lists are filled in edit mode.
mi next step is show a dropdown menu in the inspector.
with this im totally lossed.
I tried with a enum, it create the dropdown but idk how get the list values and use it for enum options, I think not is possible?.

my script so far:

using System.Collections.Generic;
using System.IO;
using UnityEditor;
using UnityEngine;
using UnityEngine.UI;

[ExecuteInEditMode] //disable this for excute only in play mode
class Dropdownfromlistv2 : MonoBehaviour
    public List<string> ListCode;
    public List<string> ListDesc;
    public Dropdown DropdowntoComplete;

    public string NombreObjetoFromList;
    public string DescrObjetoFromList;

    private void Awake()
        using var reader = new StreamReader(@"Assets\SomeFile.txt");
        while (!reader.EndOfStream)
            var line = reader.ReadLine();
            var values = line.Split(';');


    void Start()
        foreach (string t in ListCode)
            DropdowntoComplete.options.Add(new Dropdown.OptionData() { text = t });

    public void DropdownValueChanged(int value)
        NombreObjetoFromList = ListCode[value].ToString();
        DescrObjetoFromList = ListDesc[value].ToString();

I tried some basic for create a custom inspector in the same script:

public class Newcustomdropdown : Editor
    string[] _choices = new[] { "foo", "foobar" };
    int _choiceIndex = 0;
    public override void OnInspectorGUI()
        // Draw the default inspector
        if (_choiceIndex < 0)
            _choiceIndex = 0;

        _choiceIndex = EditorGUILayout.Popup(_choiceIndex, _choices);

this show me a dropdown, but eiter Idk how to asign the list values in _choices var

Can someone give me some guidance?, what is the correct/easy way to do this?.

Could this help : Enum-like menu to display list elements? - Questions & Answers - Unity Discussions ?