If you want to integrate with Marketo via the ReST API, you will want to create a custom service. Here is how:
Checkout our developers site for details on the ReST API, we also have the SOAP API if that's what you need.
You can't create a custom service if you have the Spark level of Marketo.
Create Custom Service
Under Admin click on LaunchPoint.
Under New, click New Service.
Enter a Display Name for the service. Select the API Only User previously created.
Notice that we already have native integration for popular webinar services.
Oh yeah! The service is now created, let's go ahead and get all the credentials to provide for access.