A request processor is set on a CA to manipulate and change fields in an incoming CSR according to a CA's requirements when those cannot be solved using the CA's standard settings.
Request Processors can be activated through the following vectors:
Creating a Custom Request Processor
To creatie a custom request processor:
Implement the org.cesecore.certificates.ca.ExtendedUserDataHandler interface
Compile your plugin into a JAR and make sure that JAR contains a manifest file under META-INF/services named org.cesecore.certificates.ca.ExtendedUserDataHandler which simply contains the canonical classname of your implementation. This will allow EJBCA's plugin manager to find your implementation
Make sure that your custom JAR is available on the classpath during the build phase following the steps in Creating Plugins.