integration with Zoho Creator app

I have developed a custom CRM app in Zoho Creator which I would now like to integrate with Zoho Invoice via the API so our staff can generate invoices from within our app.

We need to populate certain custom fields (e.g. "Medicare Provider No.", etc) depending on which person in our organisation is providing the services to the client.

I can't find any details in the API documentation pertaining to this. Any advice or assistance would be much appreciated.

Thanks,

Anton