How to add new user to existing database in Plesk?
To add the new MYSQL database user account and map it to the existing database follow the steps below.
Step 1: Login to Plesk panel.
Step 2: Under Websites & Domains, select Users. Click on Add New Database User.
Step 3: Adding Database User page opens. Provide the Database user name and New password. Type the new password again in the Confirm password field.
Step 4: From the dropdown, select the database for which you want to map the new user account and click OK.
Now the newly created user account "Tyler" is mapped for the existing database test_shorttutorials which is shown in the below image.