As long as they don’t provide a checkbox for that, you can use
https://inaturalist.nz/observation_fields?utf8=✓&q=has&commit=Search&order_by=name&order=asc
for the time being.
But i would like to have a checkbox for “begins with” as well:
https://forum.inaturalist.org/t/option-for-observation-field-queries-beginning-with/6373