Hey Karol, if you only want Redshift to run automatically at startup, a cronjob isn’t really necessary. Though I’m not currently using KDE, I’m sure it has a app that manages ‘startup’ applications which you can search up in application menu.
You can add Redshift to startup applications by adding a new startup program, naming it Redshift and in the command section put ‘redshift’. Keep in mind, Redshift has a few more problems across all distros I’ve tried. It’s a good app if you configure it properly.