EDIT> Searched the forum, did not find any solution, only a lot of unaswered questions, so I published this solution. My only post so far. But got flagged for a spam. We will see how it ends after I added this explanation.
TL:DR; use complications, you can change them on the watch and select weather there
You cannot simply add wather to your watch face in the Watch Face Studio, but this is the way:
Add Component Complication, for example “Short Text”
optional: choose default provider for example to Watch battery to better design your layout
Design your complication: font, sizes, the placement and size of the icon, hide Title for this complication etc.
After uploading on your watch, long press on the watch face, select customize and all these complications are selectable and you can choose many many more sources, not available in the WFS.