Send multiple key in Feign Client

You will have to use a List of String values instead of a Map.

@PostMapping(value = "nxt", consumes = MediaType.APPLICATION_FORM_URLENCODED_VALUE)
String new(@RequestParam String requestType, @RequestParam("filter") List<String> filter, @RequestBody Map<String, ?> payload);

as I found it here: Spring Cloud Feign Client @RequestParam with List parameter creates a wrong request