Greetings, im not entirely certain its possible, as it would be outside the command scope so far as i understand Roundcube, but I'm trying to make a button which essentially serves as a link to a webpage outside of the roundcube environment.
the code i have so far is:
$this->add_button(array( 'type' => 'link', 'label' => 'Anti Spam', 'command' => 'plugin.as224', 'class' => 'rlink', 'classact' => 'rlink active', 'title' => 'buttontitle', ), 'taskbar');
the command is a registered hook of init() for the plugin, although once the hook is called during click, im not certain i have any code thats capable of opening the link. 'command' seems limited to things roundcube does naturally, or its plugins wish to do.