Note:

To be able to use public S/MIME certificates from DigiCert via the iQ.Suite KeyManager, you must first have a DigiCert ONE account and have configured a CertCentral account via the Trust Lifecycle Manager. These accounts must be created by or together with DigiCert.

Information provided by GBS – see techDoc:
iQ.Suite KeyManager and the use/configuration of DigiCert S/MIME certificates