First of all, you have to check if your instance is registered or not:

[I] ❯ keybase prove --list-services

The output will be something like this:

Supported services are:
aus.social, boardgames.social, bsd.network, bugs.social, chaos.social,
cloutfla.re, comm.cx, deadinsi.de, dns, dtp-mstdn.jp, elekk.xyz,
fedi.lynnesbian.space, fosstodon.org, freeradical.zone,
functional.cafe, github, hackernews, http, https, ican.codes,
icosahedron.website, inditoot.com, is.nota.live, ischool.social,
knzk.me, koyu.space, lewd.town, masto.donte.com.br, masto.nixnet.xyz,
mastodon.librelabucm.org, mastodon.lubar.me, mastodon.sergal.org,
mastodon.social, mastodon.technology, mastodon.xyz, meemu.org,
melb.social, mstdn-bike.net, mstdn.b-shock.org, mstdn.fr,
mstdn.hogwarts.college, mstdn.io, mstdn.mimikun.jp,
mstdn.mini4wd-engineer.com, mstdn.rinsuki.net, mstdn.syui.cf,
mstdn.vodka, ne.codes, octodon.social, organicdesign.pub,
pcgamer.social, pgh.social, puppo.space, qdon.space, quey.org,
real-escape.jp, reddit, s2.libera.blue, seizemeans.com, si.pmpm.pw,
social.okoyono.de, social.privacytools.io, social.that.world,
socialnetwork.ninja, syosetu.social, takeoverthe.world,
taruntarun.net, techandbeer.social, toot.blue, toot.wales,
triplebit.net, twitter, vidja.social, web, whitespashe.uk,
witches.live, wug.fun.

This list will be longer and longer as more instances are added so maybe with grep it’s easier to figure out if it’s on the list or not:

[I] ❯ keybase prove --list-services | grep -o "quey\S\+"
quey.org,

It’s on the list

Simply call prove:

[I] ❯ keybase prove quey.org
Your username on quey.org: efertone
Please click on the following link to post to quey.org:

https://{long long url}

Checking will continue while you go authorize the proof on quey.org.
To cancel hit C-c.

Open that long url, log in if you are not logged in and basically you are done ;)

If it’s not on the list

Contact with one of the administrators of your instance.