No it goes here:
it only shows in the dropdown because I’ve used it before. You’ll have to type in “similar observation set”
Then you choose something to write in this field (that no one else would use) and put the same thing for all the observations you want to link together.
More info here: https://forum.inaturalist.org/t/using-the-field-similar-observation-set-for-linking-observations-of-lepidoptera-when-raising-on/1018