I understand... The thing is, this plugin isn’t set up for that functionality right now.
If I were you, I’d do it via events. That is, have a condition and events that request an online code, and if it’s correct, proceed with the download normally using this plugin’s plugin command call.
You can, for example, use:
https://undermax.itch.io/misterygift
That’s the best I can think of at the moment.