DOTNET CORE JWT Error converting value \"Username\" to type 'jwt_example.Model.LoginModel ERROR - api

Question

I'm trying to write an API with dotnet core 3.1 + JWT.
I created LoginModel and authController when I try to get a token
postman says to me:
"Error converting value \"Username\" to type 'jwt_example.Model.LoginModel"
That is my authController code :
public class AuthController : ControllerBase
{
private UserManager<ApplicationUser> userManger;
public AuthController(UserManager<ApplicationUser> userManger)
{
this.userManger = userManger;
}
[HttpPost]
[Route("login")]
public async Task<IActionResult> Login([FromBody] LoginModel model)
{
var user = await userManger.FindByNameAsync((model.Username).ToString());
//var user = await
if (user != null && await userManger.CheckPasswordAsync(user, model.Password))
{
var claims = new List<Claim>
{
new Claim(JwtRegisteredClaimNames.Sub, user.UserName),
new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString())
};
var signinKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes("ŞanlıFenerbahçe."));
JwtSecurityToken token = new JwtSecurityToken
(
issuer: "https://localhost:5001",
audience: "https://localhost:5001",
claims: claims,
signingCredentials: new SigningCredentials(signinKey, SecurityAlgorithms.HmacSha256),
expires: DateTime.Now.AddHours(1)
);
return Ok(new
{
token = new JwtSecurityTokenHandler().WriteToken(token),
expiration = token.ValidTo
});
}
else
{
return BadRequest(new
{
message = "Kullanıcı adı veya şifre hatalı!"
});
}
}
}
And, that is loginModel.cs:
public class LoginModel
{
[Required(ErrorMessage = "Kullanıcı adı boş geçilemez.")]
public string Username { get; set; }
[Required(ErrorMessage = "Parola boş geçilemez.")]
public string Password { get; set; }
}