you have to add custom logic, via e.g. a microflow, to copy the username to the target object’s attribute.
It is not possible to define this logic via default value definition in the domain model.
In addition, your question implies, that you want to use the username as name of an attribute, which is not possible as Mendix does not support dynamic attribute naming.
Could you just set it on creation using the value from $currentUser/Name?