Don’t ask me why they call their program that, but getting credited seems painfully indirect; you can’t just enter the valid non-cancelled confirmation code like everything else, but normally have to go through
Contact Us—>[Scroll Down to the] Early Returns [Section]—>[click on the link] “Mileage Credit” [in the context of “Request Mileage Credit for Completed Travel”]
and then enter far more detail about the trip. Of course simply permitting one to enter the confirmation code would probably make sense, at least if they aren’t reusing them, but oh well. Here’s a direct link to page for getting credit for flights already taken with Frontier Airlines.