Go to your application and switch to the Dialogflow connector tab. Here, click Add in the center of the screen or Add Dialogflow agent in the upper right corner.
The Add Dialogflow agent dialog will appear, click the Choose a file button or drag'n'drop the JSON file downloaded from GCP console and click the Add button. After the file is uploaded you will see a newly added agent in the list:
You can also visit Marketplace in the main menu to deploy an application and a test phone number that will be connected to your Dialogflow agent – to do so, click Install in the appropriate tile. Then follow the instructions to do a test call and be connected to the Dialogflow agent.
If something went wrong it could be required to delete the test application in the Applications section and the test scenario in the Scenarios section of the control panel before you try to create the test application again. Their names start with DF_ (for scenarios) or df- (for applications). Please note that the default test application can work with an agent that supports English, if your Dialogflow agent uses another language please change the test Voxengine scenario accordingly.