Используя две модели со строгим контролем типов для одного представления MVC

Если я не ошибаюсь, вам нужно добавить custom:<YOUR_ATTRIBUTE_NAME>

var poolData = {
    UserPoolId : <POOL_ID>,
    ClientId : <CLIENT_ID>,
};
var userPool = new AWSCognito.CognitoUserPool(poolData);

var attributeList = [];

var dataEmail = {
    Name : 'email',
    Value : 'email@mydomain.com'
};

var dataPhoneNumber = {
    Name : 'phone_number',
    Value : '+15555555555'
};
var grandMaName = {
    Name : 'custom:grandMaName',
    Value : 'granny'
};
var attributeEmail = new AWSCognito.CognitoUserAttribute(dataEmail);
var attributePhoneNumber = new AWSCognito.CognitoUserAttribute(dataPhoneNumber);
var attributeGrandMaName = new AWSCognito.CognitoUserAttribute(grandMaName);
attributeList.push(attributeEmail);
attributeList.push(attributePhoneNumber);
attributeList.push(grandMaName);

userPool.signUp(userData.Username, userData.Password, attributeList, null, function(err, result){
    if (err) {
        console.log(err);
        return;
    }
    cognitoUser = result.user;
    console.log('user name is ' + cognitoUser.getUsername());
    console.log('Now go to Cognito console and confirm the user.')
});