Java 8 Lambda List to Map<Int, List<String>>

I have a list of dto with following element. userSeqId have duplicate values,

private int userSeqId;
private String firstName;
private String lastName;
private String acctAgencyNumber;

I am trying to use Java 8 Lambda to group by 'userSeqId' to a Map.

I want Map<Integer, List<String>> where Key should be userSeqId and Value is List of acctAgencyNumber.

When I use

Map<Integer, List<UserBasicInfoDto>> superUserAcctMap = customerSuperUserList.stream()
    .collect(Collectors.groupingBy(UserBasicInfoDto::getUserSeqId));

I get Map<Integer, List<UserBasicInfoDto>> where key is userSeqId but value is list of whole object.