Many citation styles require the inclusion of the DOI: the unique digital object identifier that is frequently assigned to scholarly publications such as journal articles, books, and reports.
The DOI is often provided on the first page of the publication and/or in the information from the library database used to locate the article/book/report.
If you don't have the DOI, you can search for it using CrossRef at https://www.crossref.org/guestquery/.
If there is no DOI available for the work, follow the citation style guidelines for otherwise noting the source location.
If you need assistance with this, ask a librarian!