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Once you have set up your VAT within QuickBooks Online and selected the FRS system for VAT, you would use the standard VAT codes as though you were on the standard VAT scheme, and only when you actually prepare your return would the 11% FRS rate be applied and the savings calculated.
To give you some help with this as you get started, you can take a look at our Community article that we have prepared to help our customers with FRS and how it works within QuickBooks Online.
Let me know if you have any further queries.