Same question here!
On a kerberos enabled cluster, where we do not (yet) want to have (all) UIs properly (SPNEGO) kerberized, I did not find a (documented) way to get the Schema Registry working without authentication.
However i found a work around (hack), to use the registry w//out security, by adding a line
directly in following file (version used in HDF-3.2.0), before Line 101:
Plz note: I'ld prefer to get rid of that hack
but I could not find any proper Doc in the HWX HDF Docs.
The most detailed Registry Doc I found (that does not directly relate to the respective Ambari registry configs): https://registry-project.readthedocs.io/en/latest/security.html#spnego