Beside great post by Danny I found another solution with help of jquery.
Firstly I created cascading drop downs in ListB.
I made two lookup columns in ListB, Milestone and DueDate from ListB. So, both columns are visible on custom NewForm.
After this I created cascade drop downs so when user chooses Mileston, DueDate column is filtered.
Than I came across on new challenge. Default value for DueDate was None, but first/only option when clicked on drop down was DueDate of that Milestone.
So it meant I have to display a value of DueDate not None word.
I wrote a function to get first child of drop down and to display that value in additional td I created in NewForm next to td where drop down is. I hid drop down and was left with the label, as I wanted.
After this I came up a new problem, as value of DueDate that was displayed on label was in ISO format, not in format I wanted DD.MM.YYYY, that meant I have to write a new function for Date and Time convert.
After that, I got readable Date value as I needed.