In the instances where I've come across this, it has usually been in effect a "user-name", though more often consisting of a string of numbers and letters, hence my suggestion of "log-in" (with or without the addition of "code")

If you know what form it takes, you might be able to risk being more specific.

In many ways, it is redundant to add "personnalisé", since by definition that's what it's going to be! But you could always preface the term with "personal..." if you really feel it serves any purpose.