Attachment file from external source to an email C#

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C#

hi, How to attach a file(from external source) to an email.

I have a email function in C# and trying to send an attachment. If the attachment is within the project then its working. But if I upload/move the attachment to out of the project/application folder its gives error

"System.IO.IOException: The filename, directory name, or volume label syntax is incorrect. :

How do i solve it ?

What I have tried:

MimeMessage messageClient = new MimeMessage();
messageClient.From.Add(new MailboxAddress("From Name", _config.GetSection("EmailUsername").Value));
messageClient.To.Add(MailboxAddress.Parse(ClientEmail));
messageClient.Subject = CleintSubject;
BodyBuilder bodyBuilder = new BodyBuilder();
bodyBuilder.HtmlBody = ClinetMailBody;

bodyBuilder.Attachments.Add(_env.WebRootPath + "\\downloads\\sample.pdf"); ==> Working

bodyBuilder.Attachments.Add(@"https://abc.com/assets/sample.pdf"); ==> Not working

Any thoughts ?