Create a Custom Object for a Many-to-Many Structure
This example shows a Course custom object, which you'll use for many-to-many. When you're done, you'll create an intermediary object to connect it to people in your database.
For a many-to-many relationship, you don't need to create a link in the custom object. Instead, you'll add two links to the intermediary object (see below).
Click Admin, and in the tree, select Database Management > Marketo Custom Objects.
Click New Custom Object.
Enter a display name. The API Name and Plural Name populate automatically. Enter a description (optional).
You can edit these fields when you're creating them, but after they're saved, you can only edit the Plural Name field and the Show in Lead Detail slider.
Pull the Show in Lead Detail slider over to display Show if you want to view custom object data on the Lead Database page. Click Save.
The custom object information displays the content you entered. Notice that it's in a Draft state.
The next step is for you create your intermediary object (see below).
You can only populate Marketo Custom Objects via a list import, or the API.