There is no need to do this for UC students or Staff. Functionality is much better with the desktop version. But if you are brave/mad carry on...
You can use EndNote Online’s Cite While You Write (CWYW) function to create in-text citations and a bibliography in your Word document, and to format in whatever bibliographic style you choose. If you don't have EndNote desktop installed you will need to download the CWYW plug in from the Options tab in EndNote Online.
Switching to the EndNote Online application in Word
Insert Citations into a Word Document with Cite While You Write