Hi. I have had the same difficulty today, and this is the response I have got from the Xero support:
"Xero currently supports reverse charge reporting for EC purchases only. I've let our Product Team know that you'd also like us to provide support for reporting reverse charges on the sale of specified goods and services per HMRC rules. For now you'll need to enter the adjustments into Boxes 2 and 4 of you VAT return manually, and then file it with HMRC our side of Xero."