How to create mock object for a class in flutter for testing?

I am writing a bloc test for a feature in my flutter project. I need to code a demo object for a class that I could test in the BlocTest function. Below is a piece of code I wrote for it.

      blocTest<ClientBloc, ClientState>(
        'emits [ClientLoadSuccess, ClientCreateSuccess] '
        'state when successfully created client',
        setUp: () {
          PrivateClientModel client = { } as PrivateClientModel;
        },
        build: () => ClientBloc(authenticationBloc: authenticationBloc),
        act: (ClientBloc bloc) => bloc.add(
          const ClientCreateClient(375918, PrivateClientModel()),
        ),
        expect: () => <dynamic>[
          const ClientCreateSuccess(),
        ],
      );

In the above code I need to add an object of PrivateClientModel class whose code lies below. I tried doing some stuff but it did not work so asking here for a good approach.

class PrivateClientModel extends PrivateClientModelParent{
  final int id;
  final List<TestModel> activeTests;
  @JsonKey(name: 'created_at')
  final DateTime? createdAt;
  final String? businessUid;
  final String? uid;
  final String? receivingFacility;
  final String? firstName;
  final String? lastName;
  final String? username;
  final String? birthDate;
  final String? result;
  final String? ssnr;
  @JsonKey(name: 'insuranceCarrierIdentifier')
  final String? insurance;
  final String? telephoneNumber;
  final String? email;
  @JsonKey(name: 'latestPlacerOrderNumber')
  final String? placerOrderNumber;
  final String? eventNumber;
  final String? testIdentifier;
  final String? gender;
  final String? street;
  final String? houseNumber;
  final String? zipCode;
  final String? city;
  @JsonKey(name: 'country_id')
  final int? countryId;


  final int? redeemable;
  @JsonKey(name: 'next_redeemable')
  final DateTime? nextRedeemable;
  final int? atHome;
  @JsonKey(name: 'next_testable')
  final DateTime? nextTestable;
  final bool isMine;
  final bool useAuthUser;

  PrivateClientModel(
    this.id,
    this.activeTests, {
    this.createdAt,
    this.uid,
    this.businessUid,
    this.receivingFacility,
    this.firstName,
    this.lastName,
    this.username,
    this.birthDate,
    this.result,
    this.ssnr,
    this.insurance,
    this.telephoneNumber,
    this.email,
    this.placerOrderNumber,
    this.eventNumber,
    this.testIdentifier,
    this.gender,
    this.street,
    this.houseNumber,
    this.zipCode,
    this.city,
    this.countryId,
    this.redeemable,
    this.nextRedeemable,
    this.atHome,
    this.nextTestable,
    this.isMine = true,
    this.useAuthUser = false,
  });

  factory PrivateClientModel.fromJson(Map<String, dynamic> json) => _$PrivateClientModelFromJson(json);

  Map<String, dynamic> toJson() => _$PrivateClientModelToJson(this);

  PrivateClientModel copyWith({
    int? id,
    List<TestModel>? activeTests,
    DateTime? createdAt,
    String? uid,
    String? businessUid,
    String? receivingFacility,
    String? firstName,
    String? lastName,
    String? username,
    String? birthDate,
    String? result,
    String? ssnr,
    String? insurance,
    String? telephoneNumber,
    String? email,
    String? placerOrderNumber,
    String? eventNumber,
    String? testIdentifier,
    String? gender,
    String? street,
    String? houseNumber,
    String? zipCode,
    String? city,
    int? countryId,
    int? redeemable,
    DateTime? nextRedeemable,
    int? atHome,
    DateTime? nextTestable,
    bool? isMine,
    bool? useAuthUser,
  }) {
    return PrivateClientModel(
      id ?? this.id,
      activeTests ?? this.activeTests,
      createdAt: createdAt ?? this.createdAt,
      uid: uid ?? this.uid,
      businessUid: businessUid ?? this.businessUid,
      receivingFacility: receivingFacility ?? this.receivingFacility,
      firstName: firstName ?? this.firstName,
      lastName: lastName ?? this.lastName,
      username: username ?? this.username,
      birthDate: birthDate ?? this.birthDate,
      result: result ?? this.result,
      ssnr: ssnr ?? this.ssnr,
      insurance: insurance ?? this.insurance,
      telephoneNumber: telephoneNumber ?? this.telephoneNumber,
      email: email ?? this.email,
      placerOrderNumber: placerOrderNumber ?? this.placerOrderNumber,
      eventNumber: eventNumber ?? this.eventNumber,
      testIdentifier: testIdentifier ?? this.testIdentifier,
      gender: gender ?? this.gender,
      street: street ?? this.street,
      houseNumber: houseNumber ?? this.houseNumber,
      zipCode: zipCode ?? this.zipCode,
      city: city ?? this.city,
      countryId: countryId ?? this.countryId,
      redeemable: redeemable ?? this.redeemable,
      nextRedeemable: nextRedeemable ?? this.nextRedeemable,
      atHome: atHome ?? this.atHome,
      nextTestable: nextTestable ?? this.nextTestable,
      isMine: isMine ?? this.isMine,
      useAuthUser: useAuthUser ?? this.useAuthUser,
    );
  }
}

It would be really helpful if someone could help me on how I can create a sample object from this class that I could use in the test.