For other identifiers, choose Library → New entry, or click on the
New entry button, or use the keyboard shortcut
CTRL + N. In the lower part of the window, there are two boxes : "ID type" and "ID". In the field "ID type", you can select the desired identifier, e.g. "ISBN" (it works also for DOI). Then enter the identifier in the textbox below and press Enter. That will generate an entry based on the given ID (you can also click on "Generate"). The entry is added to your library and opened in the entry editor. In case an error occurs, a popup is shown.