How to pass, with a custom button, the selected items to a controller?

Question

As seen in this question (Add a custom button to a core backend component), I've created a simple plugin that adds a custom button on the com_content component.

When I click on this button, the site is redirected to the my_component controller, send task. But any of the selected items is sent to the my_component controller.

What should I do to pass the id of the selected articles to my controller?

Here is the source of my plugin:

class plgSystemSubscripcions extends JPlugin {
    public function onBeforeRender() {
        $app = JFactory::getApplication();

        if ($app->isAdmin() === true) {
            $input = $app->input;

            if (($input->getCmd('option') === 'com_content' && $input->getCmd('view', 'articles') ===  'articles') ||
                ($input->getCmd('option') === 'com_docman'  && $input->getCmd('view', 'documents') === 'documents')) {
                $toolbar = JToolbar::getInstance('toolbar');

                $url = JRoute::_('index.php?option=com_my_component&task=send&format=raw');
                $toolbar->appendButton('Link', 'mail', 'Mass email', $url);
            }
        }
    }
}

Answer 1

Your button invokes only GET action. That means all input fields are not processed, but selected items are input fields. See the same problem in this question.

Honestly I do not see any way how to pass them... If your button could submit a form that would do the trick, but as I understand it is not possible for custom buttons.