Custom fields in prestashop are called features. You have to create a feature with its corresponding values and then associate it to a specific product.
To create a feature, go to "Catalog" > "Attributes & Features", click on "Features" and create a new one by clicking on 'Add new feature'.
Enter the name of the feature and save.
Once the feature is created, click on "Add new feature value", select a feature from the drop-down menu and enter the corresponding value. Always remember to save.
To add a feature and its value to a product, we have to go to products and select the one we want to edit by clicking on it. Click on 'Add a feature', select the feature and the pre-defined value from the drop-down menu. Then save.
Finally, to include the product features in the feed, you must go to the doofinder plugin settings and enable the option.