After you've created a custom object, you need to add fields to it to meet your business needs.
Fields define the specific information used by a custom object. Link fields have a special job, to connect custom objects, and are covered in a separate article.
Click Admin, and in Database Management, select Marketo Custom Objects.
In the Fields tab, click New Field.
The three fields shown above are created automatically by Marketo when you create a custom object. Marketo manages these fields automatically and you can't edit or delete them.
Enter a display name and description.
The API name can be edited only until it's approved.
Now, choose an appropriate data type from the list.
Pull the Dedupe slider over if you want to use the new field as a unique identifier. Click Save to finish.
When you create a dedupe field, you can use it to remove duplicate fields in your database.
Add any other fields you need.
If you're building a one-to-many structure, you need to add a Link field to your custom object. For a many-to-many structure, you don't need a link field in the custom object, but you must add two link fields in the intermediary object. See Add Marketo Custom Object Link Fields to create the link fields, and Understanding Marketo Custom Objects for more information about types of custom objects.