You will have another administrative user on your checkmk host with which you can run sudo passwd siteuser to set the password.
For the ssh key you can use ssh-copy-id <your user name>@<your_checkmk_host on your source machine if it is a linux/unix system.
Otherwise you will have to add your public key to the authorized_keys file of the site user manually.
If you google ssh private/public key authentication there is tons of how tos how to achive that.