Install package from private GitHub repository with yarn - App Platform static site build fails - permission denied

Hi, my site has a dependency which is a private GitHub repository like this:

In package.json:
...
"dependencies": {
  "library": "myusername/library",
  ...
},
...

I’ve authorized Digital Ocean to access all my repositories but during the build, the library package cannot be retrieved, erroring with:

git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights

What I’ve tried:

• using different ways of adding the package detailed at https://docs.npmjs.com/cli/v7/configuring-npm/package-json#git-urls-as-dependencies
• checking if there’s a GitHub token in the process.env that I could use
• adding an .npmrc (fails with yarn)
• looked through all the pages I could find
• many variations of all of above

What is the correct way of adding this library package dependency such that it can be fetched during build?

I’d really appreciate any help.

Thank!