A registry is any object of class 52. This object allows an avatar to change their name.

You need to set a property .priceList with a list of prices for each name change. By default this is
0 0 10 20 40 80 160 320 640 1280 2560 5120 10240 20480 40960

i.e. the first two name changes are free, then 10T, then it doubles each time.

You also need to set common_flags as +1024 to instruct the client to fetch the price list.

Note: As of 2022-02-02 a minor bug (typo!) requires you to also set a property .pricelist (i.e. all lower case) if you are using anything other than the default prices. Source code has been updated, but this will take time to hit the world.