The list of custom dimensions in the Custom dropdown is all of the dimensions available in Google Analytics core reporting API which you can find here: https://developers.google.com/analytics/devguides/reporting/core/dimsmets
Once you select a dimension - such as "Product" - then the filter works just like the regex filters work in Google Analytics as well. For information on regex matching, Luna Metrics has an amazing guide that you can follow here.
If you sell desks and your top product is called "Floating Desk" then you just type in "Floating Desk" and you have now created a forecast for your top product!
For more on regex matching, Luna Metrics has an amazing guide that you can follow here.