How to set a default database server for customers' databases in Plesk?
The default database server for new subscriptions is defined in service plan settings at Service Plans > Service Plan Name > Hosting Parameters > Default Database Server. Set default database servers there and click Update & Sync.
Note: If a subscription is locked (marked with a blue lock in the Subscriptions menu), it will not be synced with its service plan. In this case, you may adjust the settings directly in subscription settings (Subscriptions > example.com > Customize).
The parameter Default Database Server in Tools & Settings determines default database servers for newly created service plans.