Question
How to allow/deny customers to choose a database server when creating a database?
Answer
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In Plesk, go to Service Plan > service_plan_name and switch to the Permissions tab. 
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Click Show more available permissions and select the Database server selection and Remote access for database users options.  
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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, adjust the settings directly in subscription settings (Subscriptions > example.com > Customize).