Login as an Administrator and go to Admin > Notification Rules > Enable the below notification under Requester Notifications and save the settings:

Send Self-service login details

This can be enabled Account Specifically by choosing the Accounts from the drop-down or you can also make use of "Assign to Account " option as shown below:

The same notification can be customized as per your needs. 

ServiceDesk Plus – MSP will send self-service login details to users who are added in the application henceforth. ServiceDesk Plus – MSP will not send the details to the users who are already created in the system.