Items is an indexed TIdFormDataField property that provides access to the form data field instances in the collection. Use AIndex to access an item in the collection by it's ordinal position in the collection. Index positions in the collection start at 0, and will not exceed the length of the collection minus 1. 

When reading the value of an object instance in the collection using AIndex, the Item is cast to a TIdFormDataField object instance. 

Items is a read-only property in TIdFormDataFields, Use Add to create a new form data field for the collection.

TIdFormDataField