Do you mean where to get the full name from the clipboard? Try OperatorID.pyUserName. As for when to set it, maybe use one of the extension activities. Do you want to set it on the creation of an assignment? If so, make use of the NewDefaults extension point activity.
Pete is correct about using OperatorID, however I would add two notes to that:
1. The OperatorID.pyUserName should not need to be set at runtime, it comes directly from the Operator record's required FullName property, I would not suggest changing it at runtime. If the FullName has not been properly entered for the operator then you could access pyFirstName and pyLastName.
2. OperatorID is a System Page (you can take a look at it in the clipboard viewer) of class Data-Admin-Operator-ID. You should add an entry for the page on the Pages and Classes tab of whatever rule you would like to access it from. Note: do not prefix the page name with a dot it is not a property but a system page, also even after doing this the autocomplete did not work for me when accessing the page (also do not use a dot in front of the page name when trying to access the page).