You wouldn't use an ASK field for this. Instead, once you've set up your mailmerge main document with all the merge fields you'll need, insert a SKIPIF field coded along the lines of:
{SKIPIF{MERGEFIELD Job_Number}<> {FILLIN "What is the customer job number to use?" \o}}
where 'Job_Number' is the name of the mergefield holding the customer job number.

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practicable to add them via any of the standard Word dialogues.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]