I'm not sure exactly what you're asking. It sounds like you're trying to authenticate to a service using a Personal Access Token or OAuth but only allow that authentication to access a single repository. If I'm understanding you correctly, authenticating with a Personal Access Token or OAuth allows you to act on behalf of a specific user. There isn't a way to "scope this down" to a single repository. If you want to only grant access to a single repository, you might want to investigate deploy keys or perhaps see if there is a GitHub App that provides the functionality you want.
Let us know if you have more questions.
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