userAttributes property
One or more custom attributes that describe the user by associating a name
with an array of values. For example, the value of an attribute named
Interests might be: "Science", "Music", "Travel". You can use these
attributes as filter criteria when you create segments. Attribute names are
case sensitive.
An attribute name can contain up to 50 characters. An attribute value can contain up to 100 characters. When you define the name of a custom attribute, avoid using the following characters: number sign (#), colon (:), question mark (?), backslash (), and slash (/). The Amazon Pinpoint console can't display attribute names that contain these characters. This restriction doesn't apply to attribute values.
Implementation
final Map<String, List<String>>? userAttributes;