You can add SSH key to the ssh-agent on different platforms discussed further.
Use the following command on Linux system to add SSH key
Open the GIT GUI and go to Help → Show SSH Key as shown in the following image.
Then, click the Copy To Clipboard button, to copy the key to the clipboard.
In Mac OS X, you can copy id_rsa.pub contents to the clipboard using the following command.
$ pbcopy < ~/.ssh/id_rsa.pub