Available from version 1.23.0 (Gerrit change 106844)
Removed in version 1.27.0
Allows modifying the internal login form when creating an account via the API.
Define function:
public static function onAddNewAccountApiForm( $apiModule, $loginForm ) { ... }
Attach hook:

In extension.json:

	"Hooks": {
		"AddNewAccountApiForm": "MyExtensionHooks::onAddNewAccountApiForm"
Called from:File(s): api/ApiCreateAccount.php

For more information about attaching hooks, see Manual:Hooks.
For examples of extensions using this hook, see Category:AddNewAccountApiForm extensions.

In MediaWiki 1.27, the hook has been removed and replaced by AuthManager functionality. Extensions which support AuthManager don't need to do anything special, API support is built in.