Мы можем использовать BodyInserters.fromFormData
для этой цели
webClient client = WebClient.builder()
.baseUrl("SOME-BASE-URL")
.defaultHeader(HttpHeaders.CONTENT_TYPE,MediaType.APPLICATION_FORM_URLENCODED_VALUE)
.build();
return client.post().uri("SOME-URI)
.body(BodyInserters.fromFormData("username", "SOME-USERNAME")
.with("password", "SONE-PASSWORD"))
.retrieve()
.bodyToFlux(SomeClass.class)
.onErrorMap(e->new MyException("messahe",e))
.blockLast();
}