You can use InlineKeyboardButton.WithLoginUrl to easily initiate a login connection to your website using the user's Telegram account credentials.
replyMarkup: new InlineKeyboardMarkup(InlineKeyboardButton.WithLoginUrl(
"login", new LoginUrl { Url = "https://yourdomain.com/url" }))You'll need to associate your website domain with your bot by sending /setdomain to @BotFather.
See official documentation about Telegram Login Widget for more information.
Server-side, you can use our separate repository Telegram.Bot.Extensions.LoginWidget
to validate the user credentials, or to generate a Javascript to show the login widget directly on your website.