Follow these steps to set a value from the Tree item to another item:

Create the following JavaScript function in the Function and Global variable declare section of the page:

function setPageItem(pvalue)
{
$s("P1_ITEM", pvalue);
}

Change the P1_ITEM to your list item.

Add a link column (‘javascript:setPageItem(“‘|| ename|| ‘”)’ As link) to your Tree view query as shown in the below example:

Select
    Case
        When Connect_By_Isleaf = 1 Then
            0
        When level = 1             Then
            1
        Else
            - 1
    End As status,
    level,
    ename   As title,
    'icon-tree-folder' As icon,
    empno   As value,
    ename   As tooltip,
    'javascript:setPageItem("'|| ename|| '")' As link
From
    emp
Start With
    mgr Is Null
Connect By
    Prior empno = mgr
Order Siblings By
    ename

The link column of the above query will set the page item P1_ITEM value with ENAME.

Please try and let me know if any issues.