Custom field in portfolio

The easiest way to add custom fields would be to use a custom fields plugin that supports custom post types, as it’s not something built into Sketch.

https://www.ads-software.com/plugins/search.php?q=custom+fields

Alternatively, if you’re comfortable with WordPress themes structure, HTML, and a bit of PHP, you could create a child theme and manually add some custom fields to it. Here’s a tutorial on custom fields you could explore:

https://www.wpbeginner.com/wp-tutorials/wordpress-custom-fields-101-tips-tricks-and-hacks/

The plugin route is more flexible, because if you decide to switch themes later to another one that supports Jetpack portfolios, your custom fields will be preserved, whereas if you put them in the theme, they won’t.

Hope this helps point you in the right direction!