Is there a token for the cron token?
The cron key is generated by Drupal core then stored as state data (system.cron_key) when Drupal is first installed. system_token_info() doesn't expose this key as a token.
You could just make a static menu link with the /cron/$KEY path as the key will remain static as long as the system.cron_key entry remains in the state table.
If the end goal is to only envoke the Backup Migrate cron handler and custom development is an option, creating your own custom route that invokes the specific backup code (e.g. backup_migrate_cron()) would be a more direct method and have a less chance of side effects from regular arbitrary runs of all cron handlers