...
<item name="order_barcode" type="order_number" pts="8" bold="true" face="xxxxxxxx" align="center" format_string="~*%s*" />
**Note: The structure of the apmconfig_receipt.xml file corresponds with the layout of the receipt from top to bottom, so if you wanted to put the barcode in as the last line on the receipt, you would put it above the first </receipt> tag
...
<item name="order_barcode" type="order_number" pts="8" bold="true" face="Barcode100" align="center" format_string="~*%s*" /> >
</receipt>