You want to search for Participants in a different LDAP. You will need to supply the tag attribute serviceMap.

Configure your Windchill to support multiple LDAP servers. Each LDAP server will be identified by a name. The name of LDAP server has to be supplied as key, and a localized display label as value. When you supply the serviceMap, you will see a dropdown in the Participant Picker with label “Service”. The selected key is sent as service parameter to the Info*Engine tasks by the Participant Picker.