Hi,
You have to insert the content into the html part of the mail you want to edit ( order_admin_notification I guess ).
"LINEVAR" is used for products in the cart so you your case, you will use "VAR" instead.
In the mail, you can read the content of the billing_address and of the shipping_address. I suppose that you want to read it from the shipping one.
So you can put in the HTML content
{VAR:shipping_address.purchase_order}
In this code I suppose that "purchase_order" is the namekey of your custom field. It is, in fact, the name of the column in the database.
Regards,