WieWaldi If your digital piano doesn't have a built-in one, get a smartphone app. Even the free ones are good enough to do the job.
Ha ha, you clearly don't understand what a dinosaur you're dealing with - I don't have a smartphone and have no desire to get one! I'm old fashioned, I have a basic phone which has just two functions; calls and texts. I deliberately chose a basic piano with no extras, because I like to keep things simple and also I never used the extras on my previous instrument. I once tried to use the built in metronome but the process to set it up and adjust the speed was confusing as the Casio did not have a screen/display attached so there was no visibility of whether it was on the desired speed or not. Tech stuff confuses me!
"Don't let's ask for the moon, we have the stars." (Final line from Now,Voyager, 1942)