You can set Link Whisper to make links to external sites open in a new tab from the Settings. This setting only applies to suggested external links, and not to manually created external links. Also, this option will only be available when you have enabled Site Interlinking.
To enable the setting, open the Settings page. Inside the “General Settings” tab, near the top, is the “Open Links to External Sites in a New Tab” option. Activate the option and save the settings to have all links to external sites open in new tabs.