I think this really depends how you generate the email subject and body, when generating the email, if you are using a server side script, you can just put the mail subject variable to the link in the mail.
For example, if you are sending the mail using php mail function, and here are the code to send it.
$to = 'nobody@example.com';
$subject = 'the subject';
//Notice that we just included the subject in $message as part of the link address.
$message = '<a href="https://xxxx?subject=${subject}">xxxx</a>';
$headers = 'From: webmaster@example.com' . "\r\n" .
'Reply-To: webmaster@example.com' . "\r\n" .
'X-Mailer: PHP/' . phpversion();
mail($to, $subject, $message, $headers);
If you are using other tools, the method will be similar.