There is nothing specific to the Sendmetric service to configure for DKIM or DMARC. We relay that data from the email you forward us. Thus any configuration for DKIM and/or DMARC will need to be done on your own sending mail server.
Depending on the mail server you are using, you will have to inject the DKIM signed header. Please consult your mail server or control panel documentation, or look at our specific mail server configurations for more information.