Downvoting makes no sense.
https://www.google.com/search?q=systemd+add+units+programmat...
You can do it either via the CLI (systemd-run), or via the D-Bus API (StartTransientUnit).
PS There's even systemd.generator for a hybrid approach.
SystemD has no dearth of programmable options.
I didnt downvote and I appreciated your reply. Just to let you know :)
I have tried available options including generators.