In smartlist filters, currently we can filter custom fields by "Contains" which states "Matches entries based on the exact whole word or phrase specified. Example: If you want to search for 'Field Value', you can search by 'Field' or 'Value'"

Can you please make substrings searchable? So if I put in "val" it will bring up all contacts that contain "val" in that custom field. Example: it returns contacts with "field value", "value", and "interval".