You are supposed to add all the authors via Mange Sources>Source manager>Create new source. There, you'd insert them as, for example:
Morales, K; Martínez, M; Mandow, C M; García-Cerezo, B M
You do not do it by editing the field code - any such edits will likely be lost immediately the field is updated. Word will take care of the 'et. al' side of things for you. That said, Word's implementation of the various referencing systems is poor. When you're done editing the document and you've finished creating citations and bibliography entries, you'll probably need to convert the bibliography, especially, to text so you can make it conform to whatever referencing system you're using.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]