@bobz I’m assuming WPMS means Multisite? Not a fan personally and not using it for years, but AFAIK it should work the same way, added once on a network level, activated for all sites. If muplugins_loaded doesn’t work for you for some reason, try plugins_loaded instead.
However, muplugins_loaded should run. See here: https://core.trac.wordpress.org/browser/tags/5.0.3/src/wp-settings.php#L295
Does you mu-plugin have a proper plugin header comment? If add_option() doesn’t work as well, I suspect your code isn’t running at all.
