To create a custom report using the Google Analytics Data API source in Airbyte, you need to provide a JSON object as a string in the Custom Reports field when setting up the connector. Here's an example of the format:

{"name": string, "dimensions": [string], "metrics": [string]}

You can find more information on creating custom reports in the [Google Analytics v4 documentation](https://docs.airbyte.com/integrations/sources/google-analytics-v4#custom-reports). For example, if you want to generate a report that shows the number of users per day and their location, you can use the following JSON object:

[{"name": "new_users_per_day", "dimensions": ["ga:date","ga:country","ga:region"], "metrics": ["ga:newUsers"]}]

Make sure to include the "ga:date" dimension in your custom report if you want to support incremental syncs. For more details on setting up the Google Analytics connector in Airbyte, refer to the [Google Analytics Data API documentation](https://docs.airbyte.com/integrations/sources/google-analytics-data-api).