This is going to depend on which version of Orchestrator you are using. As of 2020 you have the introduction of the Identity Server and configuration of External Identity Providers has been moved to the Identity Management Portal.
As you mention
web.config, this would lead me to believe you are using 2019 or earlier. If the key is not already in your configuration file, you can simple add it in.
In addition to the Identity Server as of 2020, most of the configuration keys directly related to the Orchestrator service originally stored in
web.config has been moved to
As well, in 2020, if you want access to Domain Users and Groups for your Users / Folders you’ll still be configuring
UiPath.Orchestrator.dll.config along with the mentioned External Identity Provider for the Identity Service.