Basically is configuring an external application as described in the link below, that will provide you a client/secret so you can build an implementation from your application (dialogflow in your case). You will need to read dialogflow documentation to build your implementation, UiPath’s orchestrator api reference is described on the links I shared on my previous response.
Managing External Applications (uipath.com)
Please note your dialogflow app needs to be capable of receiving an http response (provide a redirect URL)
Please take some time to understand concepts like OAuth,
OAuth 2.0 — OAuth
Couldn’t find anything specific to uipath, but here’s a generic example, please try to understand the concepts it will help you approach different use cases.
Dialogflow Tutorials: Integrate Your Dialogflow Agent with External APIs - YouTube