ASP.NET MVC getting id of selected item in list

Question

I'm creating my first app using asp.net MVC, having a very hard time. I simply want to get a single selected item in my list, press my submit button and do something with the selected item. I feel like I have all the elements here. But when I press submit I'm getting a null error in my View file (cshtml). Been all over the web and can't see how to apply what I'm finding to what I have. Any help or direction would be appreciated.

Model

    public List<string> GetFileName { get; set; }
    public List<string> ID { get; set; }
    public List<SelectListItem> FileSelect { get; set; }

public async Task ProcessRepo()
{
    string project = "https:...my url";
    string projectToken = "***";

    #region Get List of file names

    // Get list of files***********
    string uri = $"{project}tree?private_token={projectToken}&ref=master";
    string results = await client.GetStringAsync(uri);

    FileList files = new FileList();
    FileList id = new FileList();
    GetFileName = files.GetList(results);
    ID = id.GetID(results);

    #endregion
}

Controller

    public async Task<ActionResult> RepoFiles()
    {
        AssetBrowserModel _repoInfo = new AssetBrowserModel();
        await _repoInfo.ProcessRepo();
        var repoFiles = _repoInfo.GetFileName;

        //Populate list box
        var model = new AssetBrowserModel();
        model.FileSelect = new List<SelectListItem>();
        
        for (int i=0; i < _repoInfo.GetFileName.Count; i++)
        {
            model.FileSelect.Add(new SelectListItem()
            {
                Text = _repoInfo.GetFileName[i],
                Value = _repoInfo.ID[i].ToString(),
                Selected = false
            });
        }
        
        return View(model);

View cshtml

<div class="form-horizontal">
    @Html.ListBoxFor(m => m.ID, new SelectList(Model.GetFileName, "Value", "Text"),
    new { @style = "width:300px;height:100px;"})
</div>
<div>
    @using (Html.BeginForm())
    {
        @Html.HiddenFor(m => m.ID, new { id = "UserId" })<br />
        <input type="submit" name="submit" class="submit" value="Download" />
    }
</div>

FileList

class FileList
    {
        public List<string> GetList(string results)
        {
            string stringToCheck = "name";
            List<string> fileNames = new List<string>();
            string[] data = results.Split(new[] { ',', '\n' });

            for (int i = 0; i < data.Length; i++)
            {
                if (getTreeData[i].Contains(stringToCheck))
                {
                    fileNames.Add(data[i]);
                }
            }
            return fileNames;
        }

public List<string> GetID(string results)
{
    string idToCheck = "id";
    List<string> fileID = new List<string>();
    string[] data = results.Split(new[] { ',', '\n' });

    for (int i = 0; i < data.Length; i++)
    {
        if (data[i].Contains(idToCheck))
        {
            string[] newID = data[i].Split(':');
            fileID.Add(newID[1]);
            
        }
    }

    foreach(var x in fileID)
    {
        System.Diagnostics.Debug.WriteLine($"File ID is: {x}");
    }
    return fileID;
}