You need to create your post within admin_user_system_settings file as well.
A sample of this can be found from admin_user.php in the Post section.
I keep hitting this error - Undefined index: csrf
I tried adding the hidden csrf field to my admin_user_system_settings to no avail
Ok great, it got it working now thanks! The only thing is there are no error/success blocks. Do I need to alter admin_user.php to display them on update for admin_user_system_settings.php fields?
I think this is a known issue if I remember correctly since I use this on my own install-and its because the POST comes after error and success messages.
I'll work on a solution for this in 4.4.