azure marketplace deployment with existing SSHKEY prompts for existing password

Deploying a BYOL image via the web UI from Azure marketplace using an existing SSHKEY. When connecting via SSH the first thing it does is prompt you to change your password and supply the existing password. There is no existing password defined and none of the usual F5 defaults seem to work.  What's the default password?

We define a custom username and select an existing SSH key. This authenticates fine, but we get:

You are required to change your password immediately (root enforced)
Last login: Fri Nov 18 10:53:30 2022 from MYIP
Changing password for yourusername.

I can't do anything else. The admin web UI can't login either as there no known password. Setting a username and password via the deploy UI works, but we'd rather use SSH keys. What to do?