Custom fields are known as "metafields". The configuration of these fields is done in 2 steps.
Step 1: define the meta fields to be used and in which type of content you want them to appear.
Step 2: use these metafields in a specific content.
Go to your backoffice, "Configuration" > "Metafields" and select the type of content you want this custom field to refer to.
Once the content type is selected, give it a name, a description and enable the option for this field to be seen from the API requests by clicking the checkbox (this point is important). Then, save.
Once defined, go to the content type to edit it and you will see this field in the "Metafields" section. Give it a value and save.
Finally, in the plugin configuration, enable the option "Enable metafields indexing".