The auto-registration key it refers to is automatically generated when your server first starts (if not present). It's persisted to the server config, stored at different locations depending on your installation type. For a container it's at
https://hub.docker.com/r/gocd/gocd-server "Under the Hood", i.e
/godata/config/config.xml (generally for a container you will be mounting a volume to /godata so it is persisted)
When auto-generated by the server on start-up it's a UUIDv4 (generated from a secure
random number generator, represented as a string), but doesn't have to be - and is treated as a string everywhere I believe, so you can edit it yourself to something else if you would prefer something even more secure/random.