Create a file named rc.local in /etc/rc.d and make it executable. That is all there is to it (it needs to be a viable shell script of course). Do not simply create the file inside /etc as if you put it there alongside all the links to stuff in rc.d it will simply be ignored.
If you are curious, look at /usr/lib/systemd/system/rc-local.service however you don't need to monkey with anything here.
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