In the tree, select Database Management > FIeld Management and in the Marketo Custom Objects tab, click New Custom Object.
Enter a display name. The API Name and Plural Name populate automatically, but you can edit them. Enter a description (optional).
Slide the Show in Lead Detail to display Show if you want to......
view custom objects in the Lead page. Click Save.
The custom object information displays. Notice that it's in a Draft state and no fields, object names, or field names are linked to the custom object yet.
Use dedupe fields when you set up a unique Link Field for a custom object. (see below
After you've created your custom object, click the FIelds tab and New Field subtab to add fields.
Like you did with the custom object, enter a display name and description.
Easy. Now, choose an appropriate data type from the list.
Create a Link Field
When you create a custom object, you must provide a unique link field. A common unique ID is an email address, but for cars, the VIN (vehicle identification number) is perfect.
In the Fields tab, click New Field.
Name the link field, add an optional description, and be sure to select the Link data type.
Select whether the custom object is for a lead (person) or a company.
Select the field you want to connect the custom object to.
If you want to use the unique identifier to dedupe your list, pull the Dedupe slider over. Click Save to finish.
Click Custom Object Actions and select Publish Object to publish your your custom object.