When I try, it throws an error. Indeed, there's no button in the editor that seems to offer user parameters as an option. I'm currently on FME 2021.0. Thanks.
Question
When using the "OGC API - Features (WFS 3.0)" reader, is it possible to define a bbox using user parameters, rather than directly entering coords?
Try using a FeatureReader in stead. You can then manually define the BBOX coordinates as user parameters and reference them in the FeatureReader.