So, finally I found it out by myself.
Maybe there is someone out there who is also looking for this.
This is it:
1. Use a normal PC power supply; but it won't work outside the PC just like that.
2. Have a paper clip ready (or any wire)
3. Look at the 24/20 PIN adapter of the PC power supply (the biggest adapter)
4. Put the one end of the paper clip into the PIN with the green wire (i.e. PIN 14) and the other end into a PIN with a black wire.
5. Test/Turn on the power supply, it should work now (Bridge). Turn it off again. Unplug it again.
6. Cut off the end of the 12V wire of the power supply, that is the 4 PIN interface. If you are not sure go to the website of your power supply manufacturer.
Here is a picture how it looks like:
3. Connect the yellow wire of the Infill with the yellow wire of the power supply.
4. Connect also the red wire of the Infill with the yellow wire of the power supply.
5. Connect the black wire of the infill with the black wire of the power supply.
6. Start the power supply.
7. Start the Infill.
8. It should work now.
This is how it worked with mine. Hope I could help somebody, but I have to point out that you do everything on your own risk. I am not an electrician or expert in this. Don't blame me if your infill explodes.

Maybe there is some electrician out there who can just confirm that this is done right.
Best wishes
Cassidy