You will get the Advanced Filter option for the Fields to allow the portal users to filter & search the records as per the Field’s value in the customer portal.
By clicking on the Field ‘editing’ icon, you will get the Enable for Filter option to offer the filter search for that field in the customer portal.
Checkmark the “Enable for Filter” option and click on the Save button to apply the changes.
Note: Add to Filter’s List option is not for all the Data Types of the Fields. It will not be available for Lookup, Customer, Owner, Party-list type fields.