Using a key for authentication that's been generated on another device is easy!
Only three fields are required: Name, Key Type and Contents.
A friendly name for your key. The contents itself are not relevant, as it is just a user friendly representation of the key.
The type of your key. If you are unsure, feel free to try all the options - an error will be thrown if the key is of an invalid type.
The key data itself, in PEM format.
If you have a key in OpenSSH format, you may use the following command to convert it to PEM:
ssh-keygen -f ~/.ssh/id_rsa.pub -m 'PEM' -e > public.pem
Your current public key will be converted, and saved as public.pem.