If you are self-hosting a Bitwarden server, then your account email will be known to your own server, but Bitwarden’s servers will have no record of any account under that email. So you have to start by registering for a new account.
You will need to have an account with the online cloud service first.
You can then purchase the applicable subscription (individual or organization depending on your needs) via the cloud account, which will give you access to download your subscription license.
You will then upload the license to your self-hosted server to activate premium features.