Can anyone remember the Default to stop MacOS from pre-populating the username whenever an authentication dialog box is presented (ie, i want it to be blank)? I've done it before but can't for the life of me remember the Default or plist where this is located :/
I didn't think something like that was even possible. The only time I've ever seen an auth dialog be blank for the username was when the account you're running in is non-admin. If the account has admin rights the OS automatically puts in the username and requests the password. Are you sure you've been able to affect this behavior in the past with a defaults command?
If the Security Server can't find the cached creds, it will populate a dialog box, but by default on all seats with admin rights the Security Server will look for cached creds(including globally or via Smart Card)... @mm2270 is correct about if the account is a non-admin, the Security Server can't obtain the cached creds from that account, which would display a empty dialog box... But maybe THIS will be able to help with what you're trying to accomplish