Welcome to the community!
So to be clear, are you looking to execute this request more than once? If so it looks like you will want to use Collection Runner.
To start though, your Example looks mostly correct, except now for the “username” section, the value can be a postman variable, by using the curly braces, as such:
Query variable used
Once you have that in place, you can then use the Collection Runner within Postman to run against a CSV file containing different usernames.
For help with this, I would recommend more information around Collection Runner and GraphQL.
Collection Runner Docs:
Collection Runner Video:
Putting these pieces together, you should be able to accomplish what you need to.
Let me know if this helps!