This will do what you described. In this example TS_USER_05 is the user defined path field.

Sub Test_AfterPost
'On Error Resume Next

Dim td
Dim tFact
Dim tItem

Set td = TDConnection
Set tFact = TestFactory
Set tItem = tFact.Item(Test_Fields.Field(''TS_TEST_ID'').Value)

'After the drag and drop, update the user field with the path
tItem.Field(''TS_USER_05'') = Test_Fields(''TS_SUBJECT'').Value.Path
tItem.Post

'Refresh the screen
Actions.Action(''act_filter_refresh'').Execute

Set tItem = Nothing
Set tFact = Nothing
Set td = Nothing

'On Error GoTo 0
End Sub