10002 Security header is not valid
var client = new PayPalAPISoapBinding();
client.RequesterCredentials  = new CustomSecurityHeaderType();

client.RequesterCredentials = new CustomSecurityHeaderType
{
    Credentials = new UserIdPasswordType
    {
        Signature = "mysginature",
        Username = "myusername",
        Password = "mypasss"
    }
};

var request = new MassPayReq
{
    MassPayRequest = new MassPayRequestType
    {
        EmailSubject = "adsddadd",
        ReceiverType = ReceiverInfoCodeType.EmailAddress,
        MassPayItem = payments.Select(x => new MassPayRequestItemType
        {
            Amount = new BasicAmountType
            {
                currencyID = x.Currency == Currency.Euro ? CurrencyCodeType.EUR : CurrencyCodeType.USD,
                Value = x.Amount.ToString()
            },
            ReceiverEmail = x.PaypalEmailAccount,
            Note = "adasdad"
        }).ToArray(),
        Version = "61.0"
    },
};

var result = client.MassPay(request);