Defaulting an email template to a specific Action within the dropdown option

SOLVED

Hi.

I'm hoping this is something we can do.
We use an Invoice template to send invoices to customers. We would like to have the Invoice template default to a specific Action called "Invoice Sent".

I can default all actions to this selection, however it would be more appropriate to have it default for just this specific Invoice template.

Does anyone know if this is possible? 
I'm checking the workflow to see if it exist. But, maybe there is an easier method.

Let me know.

Thanks,
Stephane