Add rule on tree prompt

[khoinh]

Hi,

  I designed the tree prompt with structure Level 1 - Level 2 - Level 3. At now, I only want end-users to select nodes from Level 3. If end-users selected nodes from Level 1 or Level 2, system should show alert message and request end-users to select right nodes from Level 3. How can I do it?

Thanks
Khoi