Given a fork ghost/kodiak with a branch name add-ui, here’s how we can add commits to the branch.

git remote add ghost git@github.com:ghost/kodiak.git
git fetch ghost
git switch add-ui
# add modifications to branch.
git commit
git push

But the fork is using my main branch!

git remote add ghost git@github.com:ghost/kodiak.git
git fetch ghost
git checkout -b ghost-main ghost/main
git commit
git push