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.
Enter a display name and description.
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.
Add any other fields you need.