postInsuranceAddMemberPlans(insuranceAddMemberPlansInputRepresentation, memberId)

// Add Member Plan(s) for a Group Census Member via Connect API (simple version)

// 1) Create the top-level input
ConnectApi.InsuranceAddMemberPlansInputRepresentation input = new ConnectApi.InsuranceAddMemberPlansInputRepresentation();

// 2) Build one or more member plan entries
//    Required: contractGroupPlanId (the plan you want to add for the member)
//    Tip: You don't need to set groupCensusMemberId on each plan here because it's passed as a separate argument.
List<ConnectApi.GroupCensusMemberPlanInputRepresentation> plans = new List<ConnectApi.GroupCensusMemberPlanInputRepresentation>();

ConnectApi.GroupCensusMemberPlanInputRepresentation plan1 = new ConnectApi.GroupCensusMemberPlanInputRepresentation();
plan1.contractGroupPlanId = '0rgxx000000003kAAA'; // e.g., a2Cxx0000001234AAA
// Optional fields:
// plan1.name = 'Employee Medical Gold';
// plan1.id = 'existingMemberPlanIdIfUpdating'; // not used for add
plans.add(plan1);

ConnectApi.GroupCensusMemberPlanInputRepresentation plan2 = new ConnectApi.GroupCensusMemberPlanInputRepresentation();
plan2.contractGroupPlanId = '0rgxx000000003jAAA';
// If adding more plans, repeat and add to the list
plans.add(plan2);

// 3) Wrap plans list into the input
//    The wrapper expects a list container object
input.groupCensusMemberPlans = plans;

// 4) Execute the POST API
//    URL param is the Group Census Member Id the plans will be added to
String memberId = '0r6xx00000009CzAAI';

try {
    ConnectApi.InsuranceAddMemberPlansOutputRepresentation result =
        ConnectApi.InsuranceGroupBenefitsFamily.postInsuranceAddMemberPlans(input, memberId);

    System.debug('Add Member Plans result: ' + JSON.serializePretty(result));
} catch (Exception e) {
    System.debug('Error while adding member plans: ' + e.getMessage());
}