Change Default Custom Target to _blank

Hi there,
I have a gallery full of images that link to other websites. I would ideally like all of these links to open in a new tab, but I don’t see a place to change the default “custom target” setting that appears when I go to the information for an individual item in the gallery. In fact, I don’t see any setting that covers ALL the images in a gallery or an entire gallery all at once.

Where are the custom URL/custom target settings, and how can I force them to open in a new tab (_blank) by default?

Thanks!

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