private void WelcomeMail(string recipient)
    {
        MailMessage mailMessage = new MailMessage("MyEmail", recipient);
        StringBuilder sbEmailBody = new StringBuilder();

        sbEmailBody.Append("How can I attach .html file here instead of writing the whole code");

        mailMessage.IsBodyHtml = true;
        mailMessage.Body = sbEmailBody.ToString();
        mailMessage.Subject = "Welcome to domain.com";
        SmtpClient smtpClient = new SmtpClient("smtp.gmail.com", 587);
        smtpClient.Credentials = new System.Net.NetworkCredential()
        {
            UserName = "MyEmail",
            Password = "MyPassword"
        };
        smtpClient.EnableSsl = true;
        smtpClient.Send(mailMessage);
    }