Modiphius used to have a free pdf magazine to support their product lines, and the third issue has an STA adventure titled “The Ghost Writer” that I’ve used as an intro to the game three different times, twice in what was specifically a “learn to play” scenario. I think it’s a pretty decent little adventure that makes for a good one shot.

Also, the first time I played as a PC, the GM of that game ran the intro adventure from the core rulebook. It’s pretty good as well, though it does include the extended task mechanics which I think are useful but wouldn’t use in an intro game.