putString method
Add a string attribute for the given key.
The attribute key should be a string composed of letters, numbers or underscores ([a-z0-9_]) and can't be longer than 30 characters.
The attribute string value can't be empty or longer than 64 characters. For better results, you should trim/lowercase your strings and use slugs when possible.
Implementation
BatchEventAttributes putString(String key, String value) {
_attributes[key.toLowerCase()] = TypedAttribute(type: TypedAttributeType.string, value: value);
return this;
}