OK, I just realized that if you add a Initialize Variable step, before anything else, you need to rename the title of the step using the elipsis button.
Change the Step Title: Initialize Variable to YourVar name.
Then copy YourVar into the Name field
In the Value field, select Ask in PowerApps and then your var name will be put into Value field as YourVar_Value
Be careful, if you introduce a new Initialize var and forget the steps above then the default Initializevariable_Value will be part of your parameter set in PowerApps for that Flow.
It would be great if we could delete the default var from the Flow Editor.