var params = {
  UserPoolId: process.env.USER_POOL_ID,
  Username: 'xxx@gmail.com',
  TemporaryPassword: 'Passw0rd!'
};

var cognitoidentityserviceprovider = new AWS.CognitoIdentityServiceProvider();
cognitoidentityserviceprovider.adminCreateUser(params, function(err, data) {
  if (err) {
    callback(null, failure(err));
  }else
    var identityId =  "us-east-1:" + data.User.Username  //user sub

    var cognitoidentity = new AWS.CognitoIdentity();
    cognitoidentity.getCredentialsForIdentity(
      {"IdentityId": identityId},
      (err, credResult) => {
        if(err){
          callback(null, failure(err));
        }
        callback(null, success(credResult));
    })
});

{
  "message":"Identity 'us-east-1:8ce7ee63-d9ae-4f12-9xxxxxx' not found.",
  "code":"ResourceNotFoundException","t": "..."
}