To show the Client Email and Invoice Email in the Invoice Editor you need to create two Invoice Custom Fields:
- Entry widget: Client Email
- Entry widget: Contact Email
Invoice Custom Fields are created on the Server > Financial Tab > Invoice Customs. Make sure that the settings and names of two newly created Custom Fields are exactly the same as in the screenshots below.
Once the fields are set up they will show in the Invoice Editor and automatically link to the email fields from Contact Window: