For the SSH key for the user? This is generated via your preferred method, I used putty to generate keys for me but can use ssh-keygen etc, then just place the authorized key into the blueprint, vRA and cloud-init will place that into the machines authorized keys file at startup. This allows the user to login with the specified key rather than username/password but is not needed just something I have found most organisations prefer to meet security requirements.
how to add that ssh key. do we need to login that machine?? or we need to give host machine's ssh ???
For the SSH key for the user? This is generated via your preferred method, I used putty to generate keys for me but can use ssh-keygen etc, then just place the authorized key into the blueprint, vRA and cloud-init will place that into the machines authorized keys file at startup. This allows the user to login with the specified key rather than username/password but is not needed just something I have found most organisations prefer to meet security requirements.