How to use Deploy Keys in CI/CD?

Version: GitLab Enterprise Edition 12.3.1-ee

What I’m trying to do is push commits to a repository in the pipeline, but access to that repository requires permissions.
After a bit of querying, I think this should probably be done using Deploy Keys.

I’ve added the public key to the repository, but how do I use the private key?
This seems to need to be done by launching ssh-agent using Git Bash, but I can’t do this simply via CMD(Runner was deploy on Windows Server).
Is there a better way to handle this?