It is the plastic part on top of part CS-00132541
https://www.tefal.cz/obchod-s-prislusenstvim/Víčko-nádrže-CS-00132541/a/1800132541
Ours broke so I did a quick model, nothing fancy but it fits and works nicely. The nut is a snug fit so if you dont have your tolerances dialed in just print it a little bigger.
The temperatures of the valve might be too much over time for a PLA so maybe print with ABS if you can.