Theme_My_Login::register_extension()

public function register_extension( Theme_My_Login_Extension $extension ) {

    $this->extensions[ $extension->get_name() ] = $extension;

    /**
     * Fires after registering an extension.
     *
     * @since 7.0
     *
     * @param string                   $name   The extension name.
     * @param Theme_My_Login_Extension $extension The extension object.
     */
    do_action( 'tml_registered_extension', $extension->get_name(), $extension );

    return $extension;
}

View src/includes/class-theme-my-login.php, line 213 on GitHub