Adding to someone else’s pull request on a GitHub project that I do not own

Question

Is it possible for me to add commits to someone else’s pull request on a repository where I am not the owner?

e.g.

User A owns Project X.

User B

Answer 1

User C can add directly to User B's pull request if User B has given permission, or possibly if user C is a committer on User A's repo.

https://help.github.com/en/articles/committing-changes-to-a-pull-request-branch-created-from-a-fork

The github default permissions for users on a fork are copied from the original.

If you do have permissions your command line to add would look like this:

git push https://github.com/[userb]/[projectx].git [userc_localbranch]