With a subscription to Papers Pro, you now have the ability to add two custom fields in the metadata of your articles. For now, this feature is limited to Shared Libraries of which you are the owner, and you must have an active subscription to Papers Pro.
In order to access this feature, click on the Settings of the Shared Library you'd like to apply these custom fields to and look for the option "manage custom fields"
Once you click on "+ Add Field", you'll see the following options:
In these fields, you'll want to populate the following information:
Field name: what the name of this new field will be
CSL name: how this custom field will appear in your selected citation style; we recommend using the same name as your field name.
Section: If you'd like for your custom fields to appear in their own designated section, provide the name for that section. Otherwise, leave this space blank.
Type: how this metadata will appear. For example, here the thesis level field will provide a ticker to mark the number, while university location will provide a text field to directly type in the location.
Once you save your fields, you can view them by navigating to any article in that Shared Library and clicking here: 
You can also search your custom fields directly by searching for:
custom_metadata.{cls_name}:“{your search query}
and filling in your custom metadata.
Remember, you'll need to fill in the CSL name, not the field name.
For example, in the example above, one of our custom fields is "university location" and we have marked the location as Boston, MA. To perform a search for all articles where Boston has been listed as the "university location", we would search for:
custom_metadata.university-location:"Boston"