I want to add custom fields to show when a user selects a specific category, how is that possible?
You are able create specific custom field by forum only.
Then, if the field is not required, you are able to hide it for specific categories. Here is an example ho to do it via CSS/JS.
Check the "Show/Hide specific custom fields by selected category in the Topic form" section.
Unfortunately, you are not able to do it if the field is required.