@FormParam of jaxrs cannot pass arguments

jaxrs version: 3.0.7.Final
feign version: 11.10

I want to use @FormParam to pass arguments, and this is my code :

server:

    @POST
    @Path("/system/authentication/findGroupByUserName")
    List<Group> findGroupByUserName(@FormParam("username") String string);

client, service:

    @POST
    @Path("/system/authentication/findGroupByUserName")
    @Headers(value = {" content-type : application/x-www-form-urlencoded "})
    List<Group> findGroupByUserName(@FormParam("username") String string);

client feign builder:

  Feign.builder()
            // required values 
            .logger(new Slf4jLogger(type))
            .client(new ApacheHttpClient(HttpClientUtils.getHttpClient()))
            .contract(new JAXRSContract())
            .encoder(new JacksonEncoder())
            .decoder(new JacksonDecoder())
            .options(options);

My JAXRSContract class is copy from feign-jaxrs peoject and I wrote some code so that it can parse the @Headers annotation.
Now my issue is that the argument usename is encode to json instead of form.
I have traced the code and found the code below : feign.jackson.JacksonEncoder#encode

The data format of the form is: username = '123456', but the data format of the json is {"username":"123456"}. So how can i config the feign builder to make it work , or is there some other solutions.

Thanks !

Start with the JAXRSContract and Feign.builder configuration shown in the issue, then trace how @FormParam reaches feign.jackson.JacksonEncoder. Compare the intended application/x-www-form-urlencoded request with the encoder's output; done means the username argument is sent as form data rather than JSON, with a reproducible test or documented configuration.