@RichardP Can you provide some details about your use case? Why are you trying to connect to an Okta Service app within Workflows? If you are looking to make calls against the Okta API with OAuth, why not use the Okta connector instead? This guide may or may not help you, depending on what you’re trying to do.
Based on the doc for this connector, it appears that only Authorization Code flow is supported, not Client Credentials, and support for this flow will likely require a feature request, which you can file on the Okta Ideas Portal.