If you have a remote in your local repository named upstream that points to their repository and a remote named origin that points to your GitHub repository, you can do the following:

# Creates a local branch named `ABC_branch`
git checkout -b ABC_branch
Fetches the ABC_branch content from upstream and merges it into your local branch
git pull upstream ABC_branch

Then, if you want to have ABC_branch in your fork of their repository, you can use:

git push origin ABC_branch

I hope that helps!