If I go to the Payment File Format record for the specific format (in this case, ISO 20022...), I can indeed add the field to the ENTITY REFERENCE FIELDS section. However, when you then go to an existing record, the contents of the field aren't a hyperlink, which kinda defeats the purpose. Also, when you are creating a new record, that field is editable, which I suppose makes sense since the record hasn't been saved.
Is there additional text I can put here to make it appear as a hyperlink? Also, the link from the vendor record to the Bank Details record is already created in the background, so is "mandatory="true"" required?
<refField id="custrecord_2663_parent_vendor" label="Parent Vendor" mandatory="true" helptext="Parent Vendor"/>