If you are using static Operator ID when setting up the Web Mashup then it'll be best to circumstance the COnfirm harness. If it's dynamic one then you've to use different design approach. Guideline to circumstance is given below:
Just open your flow and go to process tab and open Confirm Harness from "If an assignment is not being performed" area. On the top right portion of Save button circumstance it using Property (generally pyUserIdentifier).
Above solution is best if you are using static operator ID and that Operator is only used for Mashup purpose.