Can anyone tell me if it's possible to make a custom url field open up in a new window when inserting the field?
Or is that done through the theme?
I've looked at some of the advanced options of the url field in MetaBox and I saw the Custom HTML5 Attributes option where I can insert a key and value. So, I tried key=target and value = _blank (and "_blank"), but, neither worked.
I'm using GeneratePress and I can easily insert the custom field and I can prepend text, but, I don't see any option to open the field in a new window.
The custom attributes help you to add the HTML attribute to the field input in the backend, not for the field value saved to the database. You can create your own shortcode to output the field value with a custom HTML attribute or contact GeneratePress support to get further assistance when outputting the field value.
Please read more on the documentation https://codex.wordpress.org/Shortcode_API