So I do exactly what @TheSevenflyer does, right down to selecting the approach for the destination in the world map. I use FSLTL and Simbrief so usually the traffic all matches up.
Sadly this is another case where Asobo tried to fix something and made it worse. Seb said in a livestream to turn this “assist” off for maximum realism, I tried that and it doesn’t work at all. If you turn it off it completely ignores STAR and approach you requested, even if the runway still matches. To get this right it should use what you ask for unless the runway doesn’t match and only then assign the new approach.
I tried with it off for a while and eventually realised it’s more realistic with it on. It’s not an assist, it’s an unbreak-me option.
But with it on you have to fully assign the SID, STAR and approach in the world map where that wasn’t really necessary before, otherwise it just doesn’t work, as you discovered.
And even if you do all of this and fly the STAR perfectly, there’s a good chance it will tell you to climb to 12,000ft when 10 miles out, or something equally daft.