If TreeView have multiple root nodes at top level wich then branch down to multiple levels.
If user click node at the deep end, how do I find root node of that branch.
I need actually string of that root node as they are all strings.

Here is a method called GetParentNode. You call it from the NodeClick method of the TreeView, passing it the key of the node that was clicked:

LPARAMETERS tcKey
*** Passed the key to a given node, returns an object reference to the parent node
LOCAL loNode, lcKey
*** Get the parent node
loNode = This.Nodes( tcKey ).Parent

*** If we do not have a parent
*** this is the root node
IF VARTYPE( loNode ) # 'O'
  RETURN This.Nodes( tcKey )
ELSE
  This.GetParentNode( loNode.Key )
ENDIF