Handling immutable prefixes in text fields

We have a web form where some of the field values have required prefixes, which identify the namespace. (This is established business logic that will not be changing.)

Example: If the user wants to create an artifact for johndoe, that artifact name will begin johndoe_ and continue with the value entered into the field.

I want to present the form in a way where it will be clear that the string johndoe_ (1) will be part of the artifact name, and (2) the prefix cannot be changed.

Here's a mock-up of the currently-implemented solution. I think this is (1) a little homely and (2) not very clear for a new user. Is there a better way to present this?