Good Morning, Hortense,
I've always found it simpler to turn off the lights for a few seconds, remove the tank top, and pour in developer quickly; I use the same method for dumping the developer, rinsing in the film washer, and pouring in the fixer. After the top is replaced, the lights go back on and everything continues in full light during each step. After the fixing is mostly complete, light is no problem anyway. Whether it's Kinderman or some other tank, the "quick-fill" cap is something of a misnomer. It's a lot easier and quicker just to pop the whole top.
Konical