If the question I wish to use as the duplicate is not offered up in the list, the Stack Exchange Android app wants me to enter a question id or URL. How do I obtain this information using the app? I don't see this information anywhere.
It is a bit tedious, but you have to go to the intended duplicate target, tap the 'Share' button and choose 'Copy to clipboard' from the menu. You can then go back to the question you intend to close, and paste the link from the clipboard into the text field where the app is asking for a question ID or URL.