Grid Plus Meta Item – Custom link : does not offer “open in new tab” option

Salut,
I want several grid thumbnails of posts to be mere links to other websites.
In the post settings I have an extra field “Grid Plus Meta Item”, including options for “Custom Link”. Here I can enter my custom link for each post and everything works fine – but there is no option to define the link as a link that should be opened in a new tab. Any hints how to solve this?