Where do I enter investment deductions?

I don't think Dexxter supports this directly. I believe that when entering your personal income tax (i.e. the codes and values provided by Dexxter) in Tax-on-web, you must use a manual code 1614 (if you are on the left) / 2614 (if you are on the right) for your investment deduction.

(Disclaimer: I am not an accountant/tax specialist, just an 'expert' based on experience who tries to make the best from it.)