Hello,
could you provide an example of what kind of commands you tried? and is it for bash or PHP or something else? I also found some docs but unfortunately, it does not say much more as these commands should be all in standard bash or PHP style https://docs.plesk.com/en-US/onyx/customer-guide/scheduling-tasks.65207/
Otherwise you may also join our Plesk Facebook community here: https://www.facebook.com/groups/plesk/ where usually people can help very quickly on such issues.
thanks
Lukas