I'm trying to link the Marketo ERP field with Salesforce. Has anyone done this before?
There is no such standard field in Marketo. It has to be a custom field.
If the field has been created in Marketo first, then you will NOT be able to map it with a salesforce field. The only way to have a Marketo field mapped with a salesforce field is to create the field in SFDC and let it sync with Marketo.