I'm putting recipe on brewersfriend and I want to add sugar at the end of boiling.
My question is in which section should I put it?
Fermentables (as late addition) or Other Ingredients (but it won't be taken into account during calculations)?
You'd want to do it as a late-addition fermentable. From the FAQ late addition entry:
"Use this to exclude the fermentable from the estimated boil gravity used in the calculator."
This means you'll have the right gravity for bittering calculations during the boil, and the right post-boil gravity (reflecting the addition of sugar).