Detect when someone adds you to a contact in telegram

Question

I want to send a message to someone when he share contact to me, this is my code:

            foreach (var update in updates.UpdateList)
            {
                switch (update)
                {
                    case UpdateNewMessage unm:
                        ...
                        break;
                    case UpdatePeerSettings ups:
                        try
                        {                        
                            if (Users.TryGetValue(ups.peer.ID, out TL.User User))
                            {
                                Contacts_Contacts contacts = await _client.Contacts_GetContacts();
                                var contact = await _client.Contacts_ImportContacts(new[] { new InputPhoneContact { phone = User.phone} });
                                await _client.SendMessageAsync(contact.users[0], "Hello");
                                if (!contacts.users.ContainsKey(ups.peer.ID))
                                {
                                    contacts.users.Add(ups.peer.ID, User);    
                                }                               
                            }
                            else
                            {
                                Contacts_Contacts contacts = await _client.Contacts_GetContacts();
                                if (!contacts.users.ContainsKey(ups.peer.ID))
                                {
                                    contacts.users.Add(ups.peer.ID, User);                                    
                                }                               
                            }
                        }
                        catch (Exception e)
                        {
                            Console.WriteLine(e.ToString());
                        }
                        break;                   
                    default:
                       break;
                   
                }
            }

but why can not add he to my contact. how can i add he to my contact programly.