Restrict user to create only one db with a specific name

Hi,

Is there a way in PostgreSQL 12 to restrict user to creating a database with a specific database name?

For example, userA can only create a database with a name called mydb. Any other names would generate an error. 

If that is not possible, will it be possible then to limit userA to creating only one database? Granting the createdb privilege would allow the user to create any  number of databases which I want to avoid.

Thanks.

Tiff