Filter by ticket custom fields, one parameter per field: filter_custom_field[FIELD]=VALUE, where FIELD is the custom field ID (the id in a ticket's custom_fields) or its name. The value depends on the type of the field. Text: words the value contains. Number: a number, or a range MIN..MAX where either end may be left out (10.. or ..20). Yes/No: true or false. Select and multi select: one option, as returned in custom_fields (ignoring case); a multi select matches the tickets that have this option among their selections. Date: YYYY-MM-DD, or a range FROM..TO where either end may be left out. Date and time: a date or date-time in the format of filter_created_at_gte, or a range FROM..TO where either end may be left out; a date alone is the whole day. URL and email: the exact value, case-sensitive. Ranges include both ends. An unknown field or a value that does not fit the type of the field returns error code 4018.