To check the hostname of the database please follow the following steps :-
1) Login to Plesk Control Panel.
2) Click on "Database"
3) You will find the Database hostname under Database server column.
Note:- By default Plesk on Grabweb server always has database defined with the local IP "127.0.0.1" as it was
on the same server where Plesk is installed but we recommend you to cross check the hostname as it may vary.