Allow me to help you on what VAT rate to use, debbie-cruxton.
Once you have set up your VAT within QuickBooks Online and selected the Flat Rate Scheme 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.
You can read through these articles for more insights about how the VAT rates work in QuickBooks: