What is the issue?

https://www.bittorrent.org/beps/bep_0053.html

I tried to test this feature and it doesn't appear implemented. This would allow you to have specific files selected after metadata is obtained.

Here's the equivalent feature for qBittorrent I think: qbittorrent/qBittorrent#7341.

It appears libtorrent, and I know anacrolix/torrent support it.

https://en.wikipedia.org/wiki/Comparison_of_BitTorrent_clients#Features_II appears to falsely claim Transmission supports it, I'm not sure why. If you can confirm it doesn't I'll update that page.

I don't think this feature is specific to any application of Transmission, it would be implemented in the generic magnet link handling somewhere, but that's not an option in the issue drop down.

Which application of Transmission?

macOS app

Which version of Transmission?

4.0.3 (6b0e49b)