Server-to-Server (JWT) .Net

As near as I can tell the answer is no, you cannot use a service account to upload content. You need to create an App User. There are many steps in the Box Portal: https://developer.box.com/docs/authentication-with-jwt

using System;
using System.IO;
using System.Threading.Tasks;
using Box.V2.Config;
using Box.V2.JWTAuth;
using Box.V2;
using Box.V2.Auth;
using Box.V2.Models;
using Box.V2.Utility;

namespace BoxPlayground
{
public class Program
{
static void Main(string[] args)
{
try
{
// Create an async method to execute and await Box SDK methods.
ExecuteMainAsync().Wait();
}
catch (Exception ex)
{
// Log any errors for debugging
Console.WriteLine(ex);
}
}

private static async Task ExecuteMainAsync()
{
// Open a stream to read and dispose of the automatically created Box configuration file.
using (FileStream fs = new FileStream($"./config.json", FileMode.Open))
{
// Use the downloaded config.json file to get a service-client
var session = new BoxJWTAuth(BoxConfig.CreateFromJsonFile(fs));
var serviceAccountClient = session.AdminClient(session.AdminToken());

// User the service client to create an "App User"
var userRequest = new BoxUserRequest() { Name = "test appuser", IsPlatformAccessOnly = true };
var appUser = await serviceAccountClient.UsersManager.CreateEnterpriseUserAsync(userRequest);

// Get the App User token and client
var userToken = session.UserToken(appUser.Id); //valid for 60 minutes so should be cached and re-used
var appUserClient = session.UserClient(userToken, appUser.Id);

// The rest of this is just example to show how to use an App User client to work with files

BoxFile newFile;

// Create request object with name and parent folder the file should be uploaded to
using (FileStream stream = new FileStream($"./BoxView.pptx", FileMode.Open))
{
BoxFileRequest req = new BoxFileRequest()
{
Name = "BoxView.pptx",
Parent = new BoxRequestEntity() { Id = "0" }
};

// Use the App User client to upload a file
newFile = await appUserClient.FilesManager.UploadAsync(req, stream);
}

var fileId = newFile.Id;

// Use the Appp User client to get a file, or in this case the download link.
var downloadUri = await appUserClient.FilesManager.GetDownloadUriAsync(fileId);
}
}
}
}