callback2.cs

[HttpGet]
public async Task<IActionResult> Callback([FromQuery]string code)
{
    var authClient = new AuthenticationApiClient("<domain>.auth0.com");
    var tokenResponse = await authClient.GetTokenAsync(new AuthorizationCodeTokenRequest
    {
        RedirectUri = "https://localhost:5001/home/auth_callbabck",
        Code = code,
        ClientId = "<your client id>",
        ClientSecret = "<your client secret>"
    });
    CustomContext.IdToken = tokenResponse.IdToken;
    CustomContext.AccessToken = tokenResponse.AccessToken;
    CustomContext.RefreshToken = tokenResponse.RefreshToken;
    return Redirect("CodeView");
}