$Config{'subject'} .= ' - ' . $Form{'some_field'}; 

Feherke.
feherke.github.io

 58  # Check Referring URL
 59  &check_url;
 60 
 61  # Retrieve Date
 62  &get_date;
 63 
 64  # Parse Form Contents
 65  &parse_form;
 66 
     # Format the subject as you wish
     $Config{'subject'} = 'Reservation ' . $Form{'TConf'};
    
 67  # Check Required Fields
 68  &check_required;
 69 
 70  # Send E-Mail
 71  &send_mail;
 72 
 73  # Return HTML Page or Redirect User
 74  &return_html; 

Feherke.
feherke.github.io

• And when you press submit, the browser sends the value of that and the other inputs to the web server.
• The web server runs formmail.pl and passes it the received form data in a pretty raw format.
• formmail.pl parses the raw form data :
  • Puts the e-mail details and configuration data in the %Config hash
  • Puts all other data in the %Form hash
  • Here I suggested to interfere and manipulate the subject
  • Checks that all necessary data was received
  • Composes the e-mail and sends it
  • Generates the result and sends it back to the browser

Feherke.
feherke.github.io

<input type="text" name="TConf" onchange="this.form.subject.value='Reservation '+this.value"> 

• Add the highlighted part to the input you want to be added to the subject
• It acts onchange, so when the input looses focus after its value was changed
• Always replaces the entire subject, so if you want to change that "Reservation" word, you have to do it here
• If the user not fills the input, then the subject will remain empty

• Go to https://jsfiddle.net/6km9yf3q/, fill the form and submit it
• Go to https://requestbin.com/r/ennu59t6qej0j/25WQvCiaIHx... and see how the form data was received

Feherke.
feherke.github.io