Is there a way in Postman to store multiple values for one key?

Your question doesn't really make sense. Postman will send the data you put, to the server. The data you put is "1,2". At the server end, if you pull the userId value and split it on the comma, you have your two values, no?

I find it incredibly unlikely that when you pull userId at the server, the value of the header is "1" and the other id has disappeared. If the web did that loads of headers (such as your gzip, deflate, br there) would get lost and stuff wouldn't work properly

In java with spring boot framework i have idea about it we have to send List userIds from request controller method this method you have to take as post method and send that data into body part with

@PostMapping("/listUsers")
public String getList(@RequestBody List<Integer> userIds) {
    // call service method
    return "page";
}


Json Request from postman
{
  1,2,3,4,5............
}

In dot net core

[Produces("application/json")]
[Route("api/accounts")]
public class AccountsController : Controller
{
    [HttpGet]
    [Route("servicesbycategoryids")]
    public IActionResult ServicesByCategoryIds([FromQuery] int[] ids)
    {
        return Ok();
    }
}