Check out and combine these 2 dev guide examples:

1. Authorization flow
2. Webhooks

See the sample code below where I added the "/subscribe" case to handle webhook subscription.

case "/test":

  String api = request.getParameter("api");

  String result = "";

  switch (api) {

    case "extension":

      result = JSON.toJSONString(rc.restapi().account().extension().list(), SerializerFeature.PrettyFormat);

      break;

    case "extension-call-log":

      result = JSON.toJSONString(rc.restapi().account().extension().calllog().list(), SerializerFeature.PrettyFormat);

      break;

    case "account-call-log":

      result = JSON.toJSONString(rc.restapi().account().calllog().list(), SerializerFeature.PrettyFormat);

      break;

  }

  response.getWriter().println("<pre>" + result + "</pre>");

  break;

case "/subscribe":

  String result = ""; 

  var eventFilters = new String[]{

         "/restapi/v1.0/account/~/extension/~/message-store/instant?type=SMS"

         }

  CreateSubscriptionRequest createSubscriptionRequest = new CreateSubscriptionRequest()

      .eventFilters(eventFilters)

      .deliveryMode( new NotificationDeliveryModeRequest()

        .transportType("WebHook")

        .address(DELIVERY_ADDRESS)

      );

  var result = rc.restapi().subscription().post(createSubscriptionRequest);