The Client Certificate screen allows you to add a client certificate for ZAP to use when testing applications protected using mutual SSL.

It is also possible to set the Client Certificate from the command line with the following switches:

-config certificate.use=true
-config certificate.pkcs12.path=/path/to/file.p12
-config certificate.pkcs12.password=WhateverThePasswordIs
-config certificate.pkcs12.index=1
-config certificate.persist=true

-config certificate.pkcs12.index is only necessary if the file contains multiple certificates and you wish to use one other than the first. Index values start at zero (0) which is the first certificate in the file.

-config certificate.persist=true is only necessary if you would like the settings persisted in ZAP's regular configuration file (so that they apply for subsequent ZAP use).