Asus tinker s enable i2s

I just installed DietPi on my Asus Tinker S Board. How can I enable the i2S Output on the GPIO Header? aplay -l only shows HDMI and USB Outputs but not the GPIO Header Output.
Before installing DietPi I used Volumio an there I had to set an overlay to activate the GPIO Header. How do I do that with DietPi?