Also see https://jazz.net/library/article/1093 lab 5 for how that works.

Some remarks:
You can only set attributes of tape Contributor/User by setting the UUID of the user. You can not search for users and get their UUID in JavaScript. You can also not search for roles. Carefully readd https://jazz.net/wiki/bin/view/Main/AttributeCustomization and https://jazz.net/library/article/1093 lab 5, the first two sections to understand what you can and can't do.

There are some default value providers available. See https://jazz.net/library/article/1093 lab 4.

There is very few information available that would allow a default value script to work. When the work item is created, there is almost nothing set, so you can't use a lot to work with. Only attributes with default value providers and the type attribute have any value.

So for the default, you can pretty much only use a single default user - which you can achieve with the available provider.