APIProxyScreen.cs

public class ProxyScreen : Screen
{
  private readonly string _Login;
  private readonly string _Pass;

  private DateTime _LastOperation;
  private Content _LastSchema;

  public ProxyScreen(string url, string login, string pass)
  {
    _Login = login;
    _Pass = pass;

    this.Url = url;
    this.CookieContainer = new System.Net.CookieContainer();

  }
  static ProxyScreen()
  {
    ServicePointManager.ServerCertificateValidationCallback += new RemoteCertificateValidationCallback(ValidateRemoteCertificate);
  }

  public LoginResult Login()
  {
    LoginResult lr = base.Login(_Login, _Pass);

    if (lr.Code != ErrorCode.OK)
      throw new Exception("Error during establishing connection to the remote site. Please check your credentials.");

    _LastOperation = DateTime.Now;
    return lr;
  }
  public new Content GetSchema()
  {
    if (EnsureLogin() || _LastSchema == null)
    {
      _LastSchema = base.GetSchema();
    }
    return _LastSchema;
  }

  public new Content[] Submit(Command[] commands)
  {
    EnsureLogin();

    Content[] graph = base.Submit(commands);
    _LastOperation = DateTime.Now;
    return graph;
  }

  private bool EnsureLogin()
  {
    if (_LastOperation == null || (((TimeSpan)(DateTime.Now - _LastOperation)).TotalMinutes > 10))
    {
      Login();
      return true;
    }
    return false;
  }

  //Callback, which is used to validate the certificate of an Acumatica ERP website in an SSL conversation
  private static bool ValidateRemoteCertificate(object sender, X509Certificate cert, X509Chain chain, SslPolicyErrors policyErrors)
  {
    //For simplicity, this callback always returns true
    //In a real integration application, you must check an SSL certificate here
    return true;
  }
}

CreateInvoiceExample.cs

class Program
{
  private static ProxyScreen context = new ProxyScreen("https://acusea.acumatica.com/Soap/AP301000.asmx", "admin@Demo", "123");
  
  static void Main(string[] args)
  {
    Content invoiceSchema = context.GetSchema();
    var commands = new Command[]
    {
      new Value { Value = "AAVENDOR", LinkedCommand = invoiceSchema.DocumentSummary.Vendor },
      new Value { Value = "123", LinkedCommand = invoiceSchema.DocumentSummary.VendorRef },
      new Value {	Value = "Demo AP Invoice", LinkedCommand = invoiceSchema.DocumentSummary.Description, Commit = true },

      invoiceSchema.DocumentDetails.ServiceCommands.NewRow,
      new Value { Value = "AACOMPUT01", LinkedCommand = invoiceSchema.DocumentDetails.InventoryID },
      new Value {	Value = "2.0", LinkedCommand = invoiceSchema.DocumentDetails.Quantity, Commit = true },

      invoiceSchema.DocumentDetails.ServiceCommands.NewRow,
      new Value { Value = "AALEGO500", LinkedCommand = invoiceSchema.DocumentDetails.InventoryID },
      new Value { Value = "1.0", LinkedCommand = invoiceSchema.DocumentDetails.Quantity, Commit = true },

      invoiceSchema.Actions.Save,

      invoiceSchema.DocumentSummary.ReferenceNbr,
      invoiceSchema.DocumentSummary.Status
    };

    var invoice = context.Submit(commands)[0];
  }
}