You want to search for Participants in a different LDAP. You need to supply the tag element directory-services.

Configure your Windchill to support multiple LDAP servers. Each LDAP server is 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 directory-services tag element and you also set display-directory-services to true, you see a drop-down in the Participant Picker with the label “Service”. The selected key is sent as service parameter to the Info*Engine tasks by the Participant Picker.