While going through the steps of this doc - Create a Custom Field for CRM Discovery- step 3 asks you to choose "Named Account" as the object for the field to be created on. Sometimes, that "Object" option is not available when creating a custom field.
The ability to select the Named Account object for a new field to be created on is a feature that must be enabled for your Marketo instance. If you come across this issue, simply submit a support case requesting that the "ABM Custom Fields" feature be enabled in your instance.
Who This Solution Applies To Customers with Account Based Marketing