:welcome:
There are 6 parts in the bar tensioner and they just drop in and are held there by 1 screw
parts 25 to 30 in the diagram on this link.
Oddly enough there does not seem to be a part number for the actual lead screw
https://www.manualowl.com/m/Stihl/MS-290-STIHL-FARM-BOSS/Manual/401004
Yep.
When installed it can't go anywhere because it is held in by the chain bar.
Usually there will be a bridge across the tensioner mechanism that stops it falling out.
These saws are a bit new for me the stuff I get in is generally 20 years old.