WordPress has the ability to assign custom fields to a post. Each custom field is handled with key/value pairs. The key is the name of the custom field. The value is the information that you want to store in that custom field.
To add a new custom field to WordPress post, you need to edit that post. In its editing page, scroll down to the area titled Custom Fields, enter text in Name, Value field. Then click Add Custom Field button to save this custom field.
If this is the first custom field on your WordPress site, you might need to click “Enter new” link to start adding the new custom field.
# There is no Custom Fields area
If you can’t see Custom Fields area, please:
+ click on Screen Options tab at top right corner of the page
+ check the Custom Fields option in the expanded area
If you can’t see the Custom Fields option, the possible reason is the post type doesn’t support custom field.
To add a new custom field for this post type, you can install a custom field plugin (for example: Advanced Custom Fields), it will help you to add a custom field to any post types easily.