I cannot seem to find the option to add a new prefix in Themaleecher.
I can set prefix in themaposter fine but not in leecher.
Can someone help?

Open themaLeecher, open themaPoster (or the other way, order doesn't matter), prefixes will get synced from themaPoster automatically on start-up.

You can as well do in themaLeecher -> "Sync" -> "themaPoster" -> "Sync prefixes" (themaPoster must be running).

Oh didn't know I had to sync, thanks.