@Path("test")
   public static class TestResource {
      
      @POST
      @Path("form")
      public String formParam(@FormParam("fp") String fp, String content) {
         return fp + "|" + content;
      }

      ResteasyClient client = new ResteasyClientBuilder().build();
      Invocation.Builder request = client.target("http://localhost:8081/test/form").request();
      Response response = request.post(Entity.entity("fp=abc", "application/x-www-form-urlencoded"));
      System.out.println("response: " + response.readEntity(String.class));

response: abc|