To remove any doubt – do not run it at 18V. There will be damage.
You can run it with a well regulated 12V supply, but a poorly regulated one may allow the peak voltage to rise too high. Unless you are an expert, it will be hard to know if this is likely.
These products are designed to run with the power supply we provide – anything else may cause damage which will not be covered by the warranty and may not be repairable.