In EasyArticles composer page, custom fields can be utilized so that authors can fill up these fields.
How To Display Custom Fields
You may refer to this Joomla custom fields documentation which explains fields in details on how to set them up in Joomla.
Once you have set up the custom fields, you can display these fields in the composer by enabling this setting at the backend of EasyArticles Settings > Editor > Enable Custom Fields.
Here is an example of how custom fields looks like on the composer.
Joomla's custom field:
EasyArticles composer display: