Take one Label use fill property to fill it. add text like "Are you sure?" take 2 buttons yes and no. Set them inside label.
Group them together and set groups visible property to variable. Let's say "varPop".
on your form's submit button instead of writing submit form code. Write
Set(varPop,true)
this will display the popup on screen when user click on submit.
On pop up's NO button write
Set(varPop,false)
so pop up will hide when user will click NO
on pop up's Yes button write your submit button code.
OnSuccess of your form write
Set(varPop,false)
so when form is successfully saved pop up will be hidden again.
