adding an alias

The WordPress plugin ActivityPub has got a great version 1. Now it is time for some publisher to move their @something@something mastodon account to their ActivityPub powered blog/fediverse.

Described by Mastodon moving followers https://docs.joinmastodon.org/user/moving/ (not al support moving) you should add your old handle as alias to your new account.

I’ve tried a couple of snippets to add an alias to the webfinger .wellknow, but haven’t had any result.
Although it is probably not that hard, but I am missing something.

How do you add an alias?