If your code is already tracked by Git then add this repository as a new repository that you can push to.
I cd to /path/to/my/already_tracked/.git
git remote set-url origin https://user:server:port/scm/ABC/my_stash_repo.git
fatal: No such remote 'origin'
also a "git remote -v " does not show the above stash-repo.
what is wrong in my thinking?
The above will only work if you've either cloned the repository or manually added a remote called origin.
If "git remote -v" doesn't show you any remotes you can simply add a remote using:
git remote add origin https://username@stash/scm/PROJECT/repo.git
The name of the remote doesn't have to be "origin" and can be any name that makes it easier for you to identify that particular remote (e.g. "stash").