Using Composer packagist to deploy C5 packages

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https://github.com/concrete5/sample_composer_package/ shows how to allow Composer to install C5 packages, and I've got it working using local composer repositories. It shows using the following composer.json file:

{
    "name": "concrete5/sample_composer_package",
    "description": "Sample package that is installable via composer",
    "type": "concrete5-package",
    ...
}


And then, at the very end, it says:

"Finally, add your git repository to a composer repository. And that's it!"

So I try it and get the following response from packagist:

The vendor is already taken by someone else. You may ask them to add your package and give you maintainership access. If they add you as a maintainer on any package in that vendor namespace, you will then be able to add new packages in that namespace. The packages already in that vendor namespace can be found at concrete5

Well, that makes sense concrete5 is not my vendor name?

Do I actually use vendor name concrete5 in my package? If so, how does this work?

Thanks