First of all, please excuse my English… My mother tongue is French.
It is unfortunately no longer possible to add a city of less than 100000 inhabitants in gnome-weather under manjaro ( Gnome 42.2, gnome-weather 42)… I think it’s a mistake since it’s still possible under Ubuntu 22.04 (Gnome 42, gnome-weather 41)
I have however found a workaround which is to indicate the location in the gnome config (dconf editor) under the two keys org/gnome/Weather/locations and org/gnome/shell/weather/locations.
Waiting for a more definitive solution…
Regards