Two examples of WebAPI with multiple Post parameters

First example

[HttpPost]
public string Post(string value1, string value2)
{
	return $"Sent: {value1}, {value2}";
}

Console.WriteLine("************* POST *************");

HttpClient httpClientPost = new HttpClient();
Task<HttpResponseMessage> httpResponseMessage = httpClientPost.PostAsync(@"https://localhost:7037/api/Values?value1=test1&value2=test2'", null);
Task<string> httpClientPostResult = httpResponseMessage.Result.Content.ReadAsStringAsync();
Console.WriteLine(httpClientPostResult.Result);

---

Second example.

First install Microsoft.AspNetCore.Mvc.NewtonsoftJson

In \WebApi\WebApi\Program.cs add line:

builder.Services.AddMvc().AddNewtonsoftJson();

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.

builder.Services.AddControllers();
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();
builder.Services.AddMvc().AddNewtonsoftJson();

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.UseAuthorization();

app.MapControllers();

app.Run();

using Microsoft.AspNetCore.Mvc;
using Newtonsoft.Json.Linq;

namespace WebApi.Controllers;

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    // POST api/<ValuesController>
    [HttpPost]
    public string Post([FromBody] JObject data)
    {
        return "test";
    }
}

using System.Text;

Console.WriteLine("************* POST *************");

HttpClient httpClientPost = new HttpClient();

Task<HttpResponseMessage> httpResponseMessage = httpClientPost.PostAsync(@"https://localhost:7037/api/Values"
    , new StringContent(@"{""additionalProp1"":[""string""],""additionalProp2"":[""string""],""additionalProp3"":[""string""]}"
        , Encoding.UTF8
        , "application/json"));

Task<string> httpClientPostResult = httpResponseMessage.Result.Content.ReadAsStringAsync();
Console.WriteLine(httpClientPostResult.Result);

Example download from here.