eval `ssh-agent`

ssh-add ~/.ssh/id_rsa

I am assuming your key file is name id_rsa

root@ubuntu-s-2vcpu-4gb-lon1-01:~# eval ssh-agent
Agent pid 1666
root@ubuntu-s-2vcpu-4gb-lon1-01:~# ssh-add ~/.ssh/idrsa
/root/.ssh/idrsa: No such file or directory

that’s what I get back

root@ubuntu-s-2vcpu-4gb-lon1-01:~# ssh-agent

SSHAUTHSOCK=/tmp/ssh-gsyd8lctjfdv/agent.12627; export SSHAUTHSOCK;
SSHAGENTPID=12628; export SSHAGENTPID;
echo Agent pid 12628;

root@ubuntu-s-2vcpu-4gb-lon1-01:~# ssh-add ~/.ssh/id_rsa

Could not open a connection to your authentication agent.

^^^^^^