You can manually enter references into the library using the References menu > New Reference command.
Please see the EndNote Help file and other Training Resources for more information on how to do this. Once the records have been added to the library, you can utilize EndNote’s Cite While You Write functionality to insert the citations into the document to replace the existing text.
https://clarivate.com/webofsciencegroup/support/endnote/
You could also try the EndNote Capture tool.
https://support.clarivate.com/Endnote/s/article/EndNote-Capture-Reference-Tool?language=en_US