Don't post comments as an answer.
Now, AFAIK, there is no custom label in report viewer control that will display any text (username here) embedded.
If you are talking of showing a username in 'report' (and not report viewer control), then you need to create a parameter in report and pass the value(username) to it at runtime in your code. This parameter can be set to display the assigned value during report viewing.
If you are looking for what I say, this resource should help:
Setting SSRS Report Parameters from ASP.NET C# Code[
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