Sign up for a free account, and implement two-factor authentication for your login. For git command access, set up an SSH key for your GitHub account. Then redirect https:// git commands to ssh in your git config:
git config --global url.ssh://git@github.com/.insteadOf https://github.com/
you don't need a credential helper with SSH access. Nor personal access tokens.
Fork the repo on GitHub. Clone it locally, and run
git remote add upstream https://github.com/ORIGINAL_OWNER/REPO_NAME.git
You can make changes, push them to your fork, and/or create a PR for upstream.
Configure your Go environment for private module downloads by setting the Go environment variable GOPRIVATE to github.com/GITHUB-USER/PRIVATE-REPO-WITH-PACKAGES
. This can also be a comma-separated list.
You can make this permanent in your Go environment by running:
go env -w GOPRIVATE=github.com/GITHUB-USER/PRIVATE-REPO-WITH-PACKAGES