1690227 is the relation id of Kt. Zürich. By adding this number to 3600000000 (=3601690227), it's possible to query an area if the corresponding relation's id is known.
[out:csv(
::id,
name,
'swisstopo:BFS_NUMMER'
;
false;
'|'
)];
area(3601690227); // Select the area of Kt. Zürich
rel[admin_level=8](area); // Find municipalities (CH: admin_level=8) within the selected area
out;
See also the map_to_area function which seems to do more or less the same.