If you want to integrate with Marketo via the REST API you will need to create an API only user. Here's how:

Under Admin, click Users & Roles. 
Click Invite New User.
Enter an Email, First Name, and Last Name for the API only user. Click Next.

 

Add an optional reason or an access expiration date. Access expiration dates are handy for short-term employees.

Select the API Only role and check the API Only checkbox. Click Next.
Click Send.

The pop-up says "An invitation is not required for API only," but that doesn't mean you've done something wrong. It just means we we'll create the role without an invite email having to be sent.

 

Alrighty then! Now let's go ahead and create the custom service.