Your 17-digit voucher number is included in the email sent by Frontier after your original booking.
Here’s how to find it:
• Check the inbox linked to your Frontier Miles account or the one used when booking
• Search for the subject line: “Your $XX voucher is here”
The email may come from:
◦ flights@emails.flyfrontier.com
◦ myfrontier@emails.flyfrontier.com
• Don’t forget to check your junk or spam folders, just in case.