A Japanese fan-site registration page, any year from the late 1980s to right now — the form barely changes. 本名: optional. ハンドルネーム: required.
A foreign visitor types their actual name into the second box, because it's the one that says "name" and the instructions are, after all, in English loanwords.
A moderator quietly fixes it before the first post goes up, the way you'd correct someone's grip on chopsticks — not unkindly, just the way everyone already knows and this one person apparently doesn't yet.
"Handle" has meant a nickname in English for a very long time, long before anyone imagined typing it into a form. The idiom "add a handle to a name" — meaning to give someone a title, a "Mister" or a "Doctor" — is recorded from 1833, and the word had drifted into meaning a nickname generally by around 1870. Ham radio operators picked it up specifically for an on-air call name; a 1938 issue of the hobbyist magazine Radio News already uses "handle" exactly the way a 2020s Discord server does.
CB radio didn't invent the word, but it is the reason everyone's grandfather knows it. The 1973 oil crisis turned Citizens Band radios into essential kit for American truckers trading fuel tips and traffic warnings, and handles spread by the thousand along the interstate — then got fixed in pop culture for good by Smokey and the Bandit (1977), Convoy (1978) and the film Citizens Band (1977). When the FCC dropped CB license and callsign requirements in 1983, the handle stopped being a nickname you used alongside your official ID and simply became the only ID there was.
Japan never actually imported the compound "handle name" from English, for the simple reason that no such compound exists in English — you just say "handle," full stop, and at least one early Japanese amateur-radio association reportedly had to say exactly that to members who kept bolting the two words together. Japan built ハンドルネーム anyway, at home, in the mid-1980s パソコン通信 (personal-computer communication) world of dial-up bulletin-board services — PC-VAN from 1986, NIFTY-Serve from 1987 — where every member chose a handoru neemu to post under instead of their real name. It worked well enough that by 1995 NIFTY-Serve alone had grown to roughly three million members, nearly all of them introducing themselves that way.
It then outlived the BBS era by flipping its own logic. On 2channel, founded by Hiroyuki Nishimura on 30 May 1999, the default stopped being "pick a name" and became "stay nameless": most posts still appear under the generic 名無しさん, nameless-san. The minority who kept a consistent handoru neemu instead earned their own label for it — 固定ハンドル, almost always clipped to コテハン — precisely because, on a board engineered for facelessness, choosing to be named at all had become the unusual move, not the ordinary one.
It's tempting to hear "handle name" and file it with the broken compounds — two English words bolted together with total confidence, the kind no native speaker would actually say. It isn't broken. It's early. When Twitter launched in 2006 and needed a word for the identity you post under instead of your real one, English reached into the exact same radio-slang drawer Japan had already been pulling from for twenty years, and came out holding the identical word: your handle. Nothing suggests either side copied the other — there's no record of the borrowing running in either direction. Two languages, a generation apart, independently decided that the right word for "who you are online, as opposed to who you are" had already been invented, by truckers, in the 1970s, and neither one needed to improve on it.