Setting up mail forwarding of an account with no Outlook license

As VasilMichev​ already mentioned, Mail Flow rules (also known as Transport Rules) will work as they work before an attempt is made to deliver the email to a mailbox.

However, this might not be the way to go in a larger corporate environment where you in general don't want to create Mail Flow rules "per user".

For most use cases, creating a Mail User would be a better fit. This allows you to create a sign-in for the user but associate it with an external email address as well. I that case, emails sent to the internal address of the user, are automatically sent (routed/forwarded) to the external address and don't require an Exchange Online or Outlook license to do so.

For more info see; Manage mail users in Exchange Online | Microsoft Learn
And: Recipients in Exchange Online | Microsoft Learn