B1UP mail subject - dunning wizard

We have SapBO and the B1UP plugin. When creating dunningmails, we would like to have the customer company name and (if possilble) the invoice number in the email subject. I imagine the last one only is possible when there is only 1 open invoice or not at all.

I tried the same variables/syntax that is used on orders (like '$[$8.0.0]') but that doesn't work in case of dunning.

I'm not an experienced SAP or B1UP user, so apologies for this. Please let me know what to set in the 'subject' field of the B1UP report action.