How to set a custom taxonomy term on a custom post type via REST JSON API?

I’m working with a custom post type (rtcl_listing) and two custom taxonomies (rtcl_category, rtcl_location). I can see the terms set on a given post (show_in_rest = true) but I cannot figure our how to update the terms to new ones via the API.

I can query the list of terms, etc via the custom tax endpoints. I just want to know how update the custom tax on a given post.