John,
Add to the beginning of the work three measures: two with quarter note (or appropriate values) beats, then one empty (for ease of later editing).
Frankly, in my opinion, it's better for the singers to SEE that there are two measures with the metronomic pulse before they begin singing (much the same for a 4-bar introduction). Even better, make the pulse pitched in the key of the piece, so they'll hear both the pulse and the key before they start.
Regards.
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