As I'm not running i3, I have not tested but only "converted" the code you provided in your post, from systemd-calls to syvinit.
But if you compare both code-snipplets and the comands on the commandline, which you shall better test first you shall see the idea.
If you get prompted for a password on the user-command line, than you would have changed something with MX Linux.
But let me kwow if the command line call, do work or not. So we would need to fix first those, if needed,I guess.