As usual you should define the communication between your windows in a generic way. For desktop I usually do an init routine that simply gives back the value so that the caller window can do it’s thing. If this is not possible for Web you could do an observer pattern or a notification. The latter usually ends up in spaghetti code while the former is a bit more work.
Add a handler method on the webpage to handle the dismissed event of the dialog. You can feed the label with the appropriate values from the dialog in this manner. Look in the language reference for addhandler.
You don’t have to use a timer with addhandler. I use it all the time with datepickers, search dialogs for document or partner search, etc. without a timer. Just don’t forget to remove the handler once the treatment of the data is completed.