Button definitions are returned by the onUserLoginButtons event handlers in plugins. By default, only system and user plugins are taken into account. The former because they are always loaded. The latter are explicitly loaded in this method.

The onUserLoginButtons event handlers must conform to the following method definition:

public function onUserLoginButtons(string $formId): array

The onUserLoginButtons event handlers must return a simple array containing 0 or more button definitions.

Each button definition is a hash array with the following keys:

• label The translation string used as the label and title of the button. Required
• id The HTML ID of the button. Required.
• tooltip (optional) The translation string used as the alt tag of the button's image
• onclick (optional) The onclick attribute, used to fire a JavaScript event. Not recommended.
• data-* (optional) Data attributes to pass verbatim. Use these and JavaScript to handle the button.
• icon (optional) A CSS class for an optional icon displayed before the label; has precedence over 'image'
• image (optional) An image path for an optional icon displayed before the label
• class (optional) CSS class(es) to be added to the button

You can find a real world implementation of the onUserLoginButtons plugin event in the system/webauthn plugin.

You can find a real world implementation of consuming the output of this method in the modules/mod_login module.

Third party developers implementing a login module or a login form in their component are strongly advised to call this method and consume its results to display additional login buttons. Not doing that means that you are not fully compatible with Joomla 4.