For security reasons, you should be very aware that accepting a remote host fingerprint automatically is a procedure that should be considered high-risk.
But if you are working with automated infrastructure or pipelines where human intervention is not possible and the constructed entities are being built in a secure fashion with guaranteed provenance, then ssh-keyscan can be used to retrieve and add the fingerprint to the known_hosts file.
ssh-keyscan -H $host >> ~/.ssh/known_hosts
Another option is to have ssh ignore the fingerprint check using the StrictHostKeyChecking option.
ssh -oStrictHostKeyChecking=no $host uptime
The StrictHostKeyChecking option can also be placed into ~/.ssh/config.
REFERENCES
askubuntu.com, automatically accepting ssh key
askubuntu.com, strict host key checking
fixyacloud, adding ssh fingerprint only if new
digitalocean, setting up ssh login keys and syntax of command when ssh-copy-id is not available
NOTES
If use of ssh-copy-id is not possible
cat ~/.ssh/id_rsa.pub | ssh username@remote_host "mkdir -p ~/.ssh && touch ~/.ssh/authorized_keys && chmod -R go= ~/.ssh && cat >> ~/.ssh/authorized_keys"