Here is a link to how the create custom fields in Marketo. https://community.marketo.com/MarketoArticle?id=kA050000000KypGCASIf you want the field to sync with Salesforce, you need...
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 ...