Prepare your Marketo account by following Custom Services to:
- Create an API Only Role with all Access API permissions.
- Create an API Only User with just that role.
- Create a Custom Service using that API Only User and find its Client ID and Secret.
When you start creating a Marketo Zap, you will be prompted to connect a New Account:
Click the button and a pop-up would appear. Enter the required user details as outlined by the screen:
Once you've entered the correct Log In details, the pop-up would disappear and you will see the Marketo account added to your Zapier account and is ready for selection: