You should be able to bracket the insert fields you wish anywhere within the Subject line or even within the message body as in the following example: <CustomerNo> inserts Acct# to our documents or <DocumentNo> inserts the Invoice # in the document, etc

Thanks, Dean. The options you have mentioned above are options that are already listed in the list of available merge fields in Electronic Message Maintenance. We need to add a field that isn't already listed. I've tried several iterations of <CustomerPONo> including using the SO_SalesOrderHeader file name in front of it but no luck getting it to actually pull the data from the field. Have you had success adding a completely new field that isn't listed in the drop down?

As I am only a user of Sage and not a programmer or support tech or Sage Rep, I am under the impression that whatever the field is named, whether it be canned field from program or USER DEFINE FIELD, can be pulled in as described.
Are you trying to pull in a 'user defined' field? Then I would believe a contact to your Sage Rep may provide you the answer as to what that field is called and then be pulled in by the bracket scenario you are already aware of.
Whoever developed or has access to designing your forms may have named those fields and should be able to tell you how they are named.

Dean,
Report output is not what is being asked about. Programmed options for inclusion within EMM are something else, and I'm curious about the answer as well. I'd doubt that this is possible, and know of a glitch when using the regular merge fields (with a KB article on the subject recommending to not use these types of fields when processing in batches).