Thanks Dan for this.
I realize that my question was not as clear and simple as I thought.
I fact, I am trying to set my Statistics Fields parameter as a list of values where the user can add new values using the 'Add Value' Button.
When the data type is defined as String I can set the property Filter as Value List and add default values.
I don't need to set the data type as Field because the values are not obtained from fields. Moreover, the available functionality is 'Add Field'. Similar but potentially misleading for the user
How do I enable this 'Add Value' button or what is the proper way to customize this intended behavior?