By default, if you insert a mailmerge field into a hyperlink field, the hyperlinks will all show the first record’s address as the 'Text to display' text. Here's how you can do get a mailmerge to display your preferred default 'Text to display' text instead:
1. Disregarding mergefield issues for the moment, insert a hyperlink into the document in the normal way, choosing whatever 'Click Here' text you want in the 'Text to display' box.
2. Select the inserted hyperlink and press Shift-F9 to expose its field code.
3. Replace everything in the field after 'HYPERLINK' with your mergefield.
4. Select the field and press F9 to update the display.

In Word 2007 & later, you can make the display text variable also, by following these additional steps:
5. Position the cursor anywhere within the display text.
6. Insert a mergefield pointing to whatever data field you want to use for the display text (this could even be the same field as used at step 3 above).
7. Delete all of the previous display text either side of your last-inserted mergefield (note that this field will likely have updated already).
8. Execute the merge.
9. After merging to a new document, use Ctrl-A, F9 to update all fields. Without this, the mergefield hover text won’t update to the correct targets.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]