When using the git clone command, you can specify the private SSH key to use by configuring the SSH agent or by directly specifying the key using the ssh-add command. Here's how you can do it:

Make sure you have the private SSH key file (.pem or .ppk) available on your local machine. If you don't have one, you can generate a new SSH key pair using tools like ssh-keygen.

Configure the SSH agent to use your private key:

1. Open a terminal or command prompt.
2. Start the SSH agent by running the following command:

eval $(ssh-agent)

3. Add your private key to the SSH agent by running the command:
ssh-add /path/to/private_key

Replace /path/to/private_key with the actual path to your private key file.

4. Now, you can use the git clone command and it will automatically use the private key added to the SSH agent:
git clone git@github.com:user/repo.git

Replace git@github.com:user/repo.git with the actual URL of the repository you want to clone.

Alternatively, you can directly specify the private key to use without relying on the SSH agent. To do this, you can set the GIT_SSH_COMMAND environment variable before running the git clone command:
GIT_SSH_COMMAND="ssh -i /path/to/private_key" git clone git@github.com:user/repo.git

Replace /path/to/private_key with the actual path to your private key file.

Remember to replace git@github.com:user/repo.git with the appropriate repository URL you want to clone.

By following these steps, you can specify the private SSH key to use when executing the git clone command.