Adding second SSH key to droplet

Ok here goes, my team member set up ssh login to our server on his computer; now I’d like to be able to access the server via my shell and I would like to be able to SSH in. The problem I am having is that I cannot seem to find a way to add my public key to the “authorized_keys” file on the server; when I try to login and run the “cat ~/.ssh/id_rsa.pub | ssh root@example.com ‘cat - >> ~/.ssh/authorized_keys’” from my local terminal i am prompted to enter a password; which i then enter the root pw which DO sent us but it consistently gives me a “permission denied” result; alternatively when I login to the server using DO’s console access page and try to run “cat id_rsa.pub >>~/.ssh/authorized_keys” it says “no such file or directory: id_rsa.pub” I’m a total beginner if you can’t tell so any help or guidance would be greatly appreciated