In the Build command input field, enter ng build --prod, and in the Publish directory input field, enter www.
Here, ng build --prod works similar to ionic build --prod and will output your Ionic app build to the www directory. This directory is outputted to because it is the build outputPath specified in angular.json of your Ionic app, unless you changed this default setting, in which case you enter the updated output directory name in the Publish directory field.
If you came here after trying to use ionic build --prod, then know I was in the same situation, twice.
Here’s a post on the Netlify Community site indicating built-in support for the ionic build command is still lacking, it may be added in the future.