Вам необходимо использовать BodyInserters.fromFormData()
для подготовки body
запроса
@Autowired
private WebTestClient client;
private WebTestClient.ResponseSpec response;
...
response = client.mutateWith(csrf()).post().uri("/login")
.contentType(MediaType.APPLICATION_FORM_URLENCODED)
.body(BodyInserters.fromFormData("username", player)
.with("password", password))
.exchange();