When you add a new Bitbucket repository, you can only enter a username/password for authentication.
If you want to use SSH, you should fallback to the generic 'Git' repository host.

SSH should be offered as an option in the Bitbucket configuration.

As an intermediate solution you can add a descriptive text warning for the potential insecurity of plaintext credentials, and suggest to use 'Git' repository host with SSH instead.
This can also be added to the documentation.