Yep you’re right.
CurrentDocument on NotesUIWorkSpace will give you the open document on the screen but not if you are in an other frame.
I suggest that you work in the Inbox view/folder with a DialogBox called by an action button. Your custom form would be then display in your dialogBox.
From the Inbox view/folder, you could know wich documents are selected with the Documents method of NotesUIView class
set uiview = NotesUIWorkSpace.CurrenView
set docCollection = uiview.Documents
Then you could iterate in all the documents in the collection in order to add your tag field with what was inputed in your DialogBox
Whay do you think ? Hope you’ll understand, sorry for my poor english