Hi @benenoo,
I understand that you’re trying to add a custom field to your Single Page Layout. If the field is added to your Add Listing Page, you can easily add it to your Single Listing Page by going to Directory Builder > Single Page Layout > Contents. The field should be available on the right side, drag and drop it to the left section.
If you want it to be added in the header, it won’t be possible, unfortunately.
Regards,
Mahdi.