A bit late, I know...
But if anyone still wants to add PHP 7.4 to Plesk Onyx, it is possible in 5 minutes max.
1) If not present yet, install PHP 7.4 on your server (procedure easily found anywhere on the Net).
2) Add PHP 7.4 to Plesk.
On my server (Debian 9.11), I found:
Therefore I sent...