I assume that you have a Form with 3 Fields ( Value 1, Value 2 and Sum )
Set an OnEnter Event with call microflow.
In this microflow change Sum to Value 1 + Value 2.
You will find in the Forum more elegent and bether ways, just google "sum of integer mendix"
Hope this Helps
Iv're tried this, but sum attribute is only taking the value 2 :
this event for both values input:
And it's not working!