API de WhatsApp con Java

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.time.Duration;

public class EnviarWhatsapp {
    public static void main(String[] args) throws Exception {
        String json = "{"
            + "\"api_key\":\"" + System.getenv("WSP_API_KEY") + "\","
            + "\"sender\":\"51999888777\","
            + "\"number\":\"51987654321\","
            + "\"message\":\"Hola desde Java\""
            + "}";

        HttpRequest peticion = HttpRequest.newBuilder()
            .uri(URI.create("https://api.kontacto.pro/send-message"))
            .timeout(Duration.ofSeconds(30))
            .header("Content-Type", "application/json")
            .POST(HttpRequest.BodyPublishers.ofString(json))
            .build();

        HttpResponse<String> respuesta = HttpClient.newHttpClient()
            .send(peticion, HttpResponse.BodyHandlers.ofString());

        System.out.println(respuesta.statusCode() + " " + respuesta.body());
    }
}

RestClient cliente = RestClient.create("https://api.kontacto.pro");

Map<String, Object> respuesta = cliente.post()
    .uri("/send-message")
    .contentType(MediaType.APPLICATION_JSON)
    .body(Map.of(
        "api_key", apiKey,
        "sender", "51999888777",
        "number", "51987654321",
        "message", "Hola desde Spring Boot"))
    .retrieve()
    .body(new ParameterizedTypeReference<>() {});