To add new or edit Custom Content Fields please go to Wp-Admin Dashboard > Classiads Option > Content Fields here you can add or edit Content Fields.
How to Add Custom content fields in Listing submit page.
If you will check the On Listings page as in below mentioned image then this content fields will be shown on Listing Submit Page
How to Add Custom content fields in Search Form.
If you will check on “Search by this Fields” then this content fields will be shown on Search Form. You can also assign this content field to Selective or Selected Categories.
For more options please see the attached image
How to add Sub Content fields in Content Fields
Go to Wp-Admin Dashboard > Content Field > Hover the mouse on your desire Content field > Configure > here you can add Sub-content fields.
Hope this Article will help you