If this was an Airbus, it will activate the approach mode on it's own only when it has an LNAV route all the way to the runway (at least in the terminal area).
That is not very common. When you are getting radar vectors to an approach you do it by selecting perf page and selecting activate approach mode and you then confirm it. That will give you managed speeds for the approach. If you forget you will get speed 250 when you try to select managed speed.