Missing control knob for bathroom radiator
This is a knob for a potentiometer, you should grab a caliper and measure the dimensions of the axis, and the internal dimensions of the hole. This will be a bit complex because you need something like this:



See the bit of plastic that sticks out around the back, that takes the axis. This is uncommon, most knobs would be like this:



...without the extra bit sticking out in the back. You can use one like this too, if the diameter fits inside the smaller hole around the axis, this could be easier to find.

Try "boutons pour potentiomètre" or "potentiometer knob" on aliexpress or reichelt...