treeview item id

first you need to get the id of the tree you selected:
var %y = $gettok($did(<id>,1),4,32)



then set the changebranch to that tree,
did -i <dialog> <id> 1 cb root %y



now that mdx knows that we are on that branch simply use $gettok again to get the id of the selected item.
$gettok($did(<id>,1),5,32)