It used to be possible to put the article's URL in the general tab of the entry editor. It's not there now, and there's no place else to enter it. But for entries created with JabRef 4, there is often an "Other fields" tab, where the URL already entered is shown.
JabRef 5.0-dev--snapshot--2019-05-06--master--9b903abbf
Windows 10 10.0 amd64
Java 1.8.0_201
It used to be possible to put the article's URL in the general tab of the entry editor. It's not there now, and there's no place else to enter it. But for entries created with JabRef 4, there is often an "Other fields" tab, where the URL already entered is shown.
JabRef 5.0-dev--snapshot--2019-05-06--master--9b903abbf
Windows 10 10.0 amd64
Java 1.8.0_201