Unless I have missed a very recent, very major development, you can't enter a discount code on any Sixt website.
The reason is simple: contrary to most other providers, Sixt web rates are not calculated on the basis of a full-price rate minus a web discount. Web rates work totally separately from the office/telephone rates, which are the ones you can save money on with your discount code(s) and/or shiny card(s).
No matter what sort of discount you are entitled to, always always check the web rates before agreeing to a rental.