Pam,

I think what you are asking is impossible if you mean that it is an
AutoNumber data type.  As soon as the Authorisation field, or any other
field, has data entered into it, a value will be allocated to the
Autonumber field.  However, you could make it a Number data type, and
use a VBA procedure to allocate the value, for example on the
AfterUpdate event of Authorisation.  You would also need to take steps
to ensure that none of the fields from the Item Table table can be
accessed until after the PO# has a value allocated.

As a side issue, unrelated to your question, it is not a good idea to
use a # as part of the name of a field or control, and I would recommend
changing this.