How can I add an already generated SSH key to git bash?

On windows you might need to start the ssh agent like this

# start the ssh-agent in the background
$ eval $(ssh-agent -s)
> Agent pid 59566

Add your SSH private key to the ssh-agent. If you created your key with a different name, or if you are adding an existing key that has a different name, replace id_rsa in the command with the name of your private key file.

$ ssh-add <path/to/key>

Got this information from here under “Adding your SSH key to the ssh-agent”:
https://help.github.com/en/articles/generating-a-new-ssh-key-and-adding-it-to-the-ssh-agent#adding-your-ssh-key-to-the-ssh-agent