A trigger that happens when an entry is opened can interact with the used in only a limited fashion. The Memento JavaScript message() global function may be called.
On a mobile device, this merely displays the value passed to it at the bottom of the screen for 2 or 3 seconds, when it then disappears. If you field is called "My text", the statement would be message(entry().field("My text"));
On a computer, I believe a pop-up panel is opened with the contents of the "My text" field in it, and I think it stays until closed by the used. I haven't used Memento on a computer very much, so I'm not sure.