public async Task <AccessTokenResponse> RequestAccessTokenAsync(AccessTokenRequest payload)
{
Signature signatureService = GetSignatureService();
string messageSignature = signatureService.Sign(JsonConvert.SerializeObject(payload));
return(await authApi.AuthenticationAccessTokensPostAsync(
CONTENT_TYPE,
signatureService.ClientRequestId,
GetApiKey(),
signatureService.TimeStamp,
payload,
messageSignature
));
}