How to use the ask-sdk-core.DefaultApiClient function in ask-sdk-core

GlobalHandlers.SessionEndedRequestHandler,
    UseApiRequestHandler, // API sample

    PurchaseHandlers.WhatCanIBuyIntentHandler, // purchase handlers
    PurchaseHandlers.TellMeMoreAboutProductIntentHandler,
    PurchaseHandlers.BuyIntentHandler,
    PurchaseHandlers.BuyResponseHandler,
    PurchaseHandlers.PurchaseHistoryIntentHandler,
    PurchaseHandlers.RefundProductIntentHandler,
    PurchaseHandlers.CancelProductResponseHandler,

    GlobalHandlers.IntentReflectorHandler, // last
  )
  .addRequestInterceptors(myLocalizationInterceptor, initPurchaseHandlers) // lang & purchase
  .addErrorHandlers(GlobalHandlers.ErrorHandler)
  .withApiClient(new Alexa.DefaultApiClient()) // API to get user permissions and in-skill purchases
  .lambda();

const skillBuilder = Alexa.SkillBuilders.custom();

exports.handler = skillBuilder
  .addRequestHandlers(
    LaunchRequestHandler,
    CreateReminderHandler,
    SessionEndedRequestHandler,
    HelpHandler,
    CancelStopHandler,
    UnhandledHandler,
  )
  .addRequestInterceptors(RequestLog)
  .addResponseInterceptors(ResponseLog)
  .addErrorHandlers(ErrorHandler)
  .withApiClient(new Alexa.DefaultApiClient())
  .withCustomUserAgent('cookbook/reminders/v1')
  .lambda();

const skillBuilder = Alexa.SkillBuilders.custom();

exports.handler = skillBuilder
  .addRequestHandlers(
    LaunchRequest,
    NameIntent,
    EmailIntent,
    NumberIntent,
    SessionEndedRequest,
    HelpIntent,
    CancelIntent,
    StopIntent,
    UnhandledIntent,
  )
  .addErrorHandlers(ProfileError)
  .withApiClient(new Alexa.DefaultApiClient())
  .withCustomUserAgent('cookbook/customer-profile/v1')
  .lambda();

},
};

const skillBuilder = Alexa.SkillBuilders.custom();

exports.handler = skillBuilder
  .addRequestHandlers(
    LaunchRequest,
    SessionEndedRequest,
    HelpIntent,
    CancelIntent,
    StopIntent,
    UnhandledIntent,
  )
  .addErrorHandlers(SettingsError)
  .withApiClient(new Alexa.DefaultApiClient())
  .withCustomUserAgent('cookbook/device-settings/v1')
  .lambda();

};

const skillBuilder = Alexa.SkillBuilders.custom();

exports.handler = skillBuilder
  .addRequestHandlers(
    LaunchRequest,
    GetAddressIntent,
    SessionEndedRequest,
    HelpIntent,
    CancelIntent,
    StopIntent,
    UnhandledIntent,
  )
  .addErrorHandlers(GetAddressError)
  .withApiClient(new Alexa.DefaultApiClient())
  .withCustomUserAgent('cookbook/device-location/v1')
  .lambda();