Here is a link to how the create custom fields in Marketo. https://community.marketo.com/MarketoArticle?id=kA050000000KypGCAS
If you want the field to sync with Salesforce, you need to create the same custom field in your Salesforce instance.
A
Anonymous
February 10, 2015
How do I create that field in Marketo's Field Management? And how do I ensure that it shows up on salesforce? (i already have the marketo/sfdc sync working)
You need to create the field and values in the Field Management area in the Admin section. Once that's created, you can click the "+" on your form and search the field name to add to your form.