If you want to integrate with Marketo via the ReST API, best practice is to create an API only user role, Create an API Only User and then a Create a Custom Service for Use with ReST API. Here is how to create the API only user role:
Under Admin, click on Users & Roles.
Click on the Roles tab and select New Role.
Enter a Role Name, select the API access Permissions you want to give and click Create.
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OK, well done! Now let's Create an API Only User.