Writing UsernameCreate attribute in message object?

Hello,

my macro writes messages and while the basic function works fine I wanted to use the attribute "aucAttrUsernameCreate" to store the username of the current user.
But when I try to set a value is get an error saying the "read only" flag is set.
I tried to set ReadOnly to false for the attribute, but then i get "Can't assign to read-only property".
What is this attribute for and is it possible to write a value to it?

Best regards

Stefan