In Owncloud create a "Joplin" folder off the root and add that to the webdav file path. It doesn't have to be "Joplin" but at least it makes it clear what the folder is for...
This is mentioned in the synchronisation instructions.
Then input the WebDAV URL (to get it, click on Settings in the bottom left corner of the page, in Nextcloud), this is normally
https://example.com/nextcloud/remote.php/webdav/Joplin( make sure to create the "Joplin" directory in Nextcloud ), and set the username and password.
It mentions Nextcloud but also applies to Owncloud from which Nextcloud was forked.
Make a JEX file as a backup (Files > Export > JEX - Joplin Export File) just to be safe. Then copy all the Joplin data from the Owncloud root into the Owncloud Joplin folder before you sync with the new path. If you do not, Joplin will find an empty webdav folder and try to delete your notes from the client and you will likely get a "fail-safe" warning. When all is working you can delete the old Joplin sync files from the Owncloud root taking care not to also delete the Joplin folder you just created!!!.