Setting subject of email using the Reply-to Box

Summary:

I know it is possible to create a mailto: link with a custom subject line that gets appended to the email when someone clicks on it in the body of the email by doing something like, "example@example.com?Subject=Reply to this email" (some suggest using %20 for spaces). This is usually also placed in the <a href= tag.

But I have tried unsuccessfully to use the same concept to try and append a subject line to the reply-to box in the emails settings when creating the email so that it can be applied universally to anyone who would click the "reply" button as opposed to clicking a link in the body of the email. This method gives me a constant error when trying to save, which is preventing the email from saving with the code - i.e. it doesn't work that way in this scenario. Is there a way to add a subject line to the mail-to in