Add Input Field missing, python toolbox field mapping parameter

[JasonXue]

I set up a field mapping parameter in my python toolbox pyt file, and add a table for it. Now I need to add input table field for each of target fields like we do this in Appending tool. However, there is no 'Add input field' option when I right click that. How can I make this option available in python toolbox.

Append Tool has this option: