The grayed-out Insert Merge Field button tells me this isn't a true mailmerge main document and the mergefields are just being used as placeholders (a fairly common misuse of mergefields).
As for the field merely outputting «Service_AS», that's to be expected for your document before your C# code runs. What do you see after it has run?
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Cheers,
Paul Edstein
[Fmr MS MVP - Word]
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