@Override
    public ResponseEntity<LoginResponse> login(RequestEntity<LoginRequest> loginRequest) {
        restTemplate.setMessageConverters(Arrays.asList(new CustomHttpMessageConverter()));
        return restTemplate.exchange(loginUrl, HttpMethod.POST, loginRequest, LoginResponse.class);
    

@Data
public class LoginRequest<R extends LoginResponse> {

    private Long login;

    private Long password;

    @Data
    public static class LoginResponse {

        private int requestSign;

    }
}

public class CustomHttpMessageConverter extends AbstractHttpMessageConverter<LoginRequest> {

    public GateHttpMessageConverter() {
        super(new MediaType(MediaType.TEXT_PLAIN, StandardCharsets.UTF_8));
    }

    @Override
    protected boolean supports(Class<?> clazz) {
        return true;
    }

    @Override
    protected LoginRequest readInternal(Class<? extends LoginRequest> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
        String result = new BufferedReader(new InputStreamReader(inputMessage.getBody()))
                .lines().collect(Collectors.joining("\n"));
        
       String[] ar = result.split("&");
        
        long nextExtTrnId = 1;
        int requestSign = 0;

        LoginRequest loginRequest = new LoginRequest();
        
        LoginRequest.LoginResponse response = new LoginResponse();
        response.setNextExtTrnId(nextExtTrnId);
        response.setRequestSign(requestSign);
        
        return loginRequest;

    }

    @Override
    protected void writeInternal(LoginRequest loginRequest, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        String body = String.format("LOGIN=%s&PASSWORD=%s", loginRequest.getLogin(),loginRequest.getPassword());
        outputMessage.getBody().write(body.getBytes());
        
    }
}