api/API_sm/Controllers/AddClient.cs

using API_sm.BaseConnection;
using API_sm.ModelBase;
using Microsoft.AspNetCore.Mvc;

namespace API_sm.Controllers
{
    [Route("api/[controller]")]
    [ApiController]
    [ApiExplorerSettings(GroupName = "Добавление")]
    public class AddClient : ControllerBase
    {
        ApplicationContext db = new ApplicationContext();
        [HttpPost]
        public IActionResult Index(Client client)
        {
            db.Add(client);
            db.SaveChanges();
            return Ok(client);
        }
    }
}