How do you make an alias or function that retains tab completion?

First find out which complete-function is used for the systemctl command:

complete | grep " systemctl$"

The output looks like this:

complete -F _functionname systemctl

Then use:

complete -F _functionname sctl

To register the function for the completion of your alias.

Now, when you type sctl <tab><tab>, the same suggestions as when you type systemctl will appear.