See https://forums.zotero.org/discussion/71081/bug-report-zotero-in-libreoffice-does-not-play-nice-with-the-notebookbar-uis

Possible to make this happen for LibreOffice 6.4, although hopefully Document Foundation will add something that will create a default tab for an extension without needing anything on our part. Seems like 6.3.6 is scheduled to be released for 2020 May, so no movement till then.