The field construction would be:
{IF«PrefName» = "" «Fname» «PrefName»}

Note: The field brace pairs (ie '{ }') for the above example are created in the document via Ctrl-F9 (Cmd-F9 on a Mac) - you can't simply type them or copy & paste them from this message. Likewise, you can't type or copy & paste the chevrons (ie '« »') - they're part of the actual mergefields, which you can insert from the mailmerge toolbar.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]