A short guide: https://forum.inaturalist.org/t/how-to-use-inaturalists-search-urls-wiki-part-2-of-2/18792
Some examples with Host Plant ID field:
https://www.inaturalist.org/observations?field:Host%20Plant%20ID=Pinus
https://www.inaturalist.org/observations?place_id=any&subview=table&taxon_id=47201&field:Host%20Plant%20ID=Quercus
So, it is enough to add to the query (? or &) a variable like “field:Host%20Plant%20ID=[name of the plant]” (%20 code = space).