Webflow can’t pull a form field into the notification email subject, it only uses the form name.
Only workaround is sending the form through Zapier/Make to build a custom subject (had to do this on a client build).
These are the form variables that Webflow seems to allow currently for us to map in the form submission email notification.
If you want more control and customization, you can probably take the automation route as suggested above / explore different form providers if that fits for your project.