Is this for accessing Tableau Server's Postgres Repository? The username is fixed and the password needs to be set using a tabadmin command. Check out this online help article for more info: Collect Data with the Tableau Server Repository
Hope this helps!
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You need to enable access to the postgres DB using Tabadmin
tabadmin dbpass -u readonly <password>
Once you have access you can connect with Tableau desktop
server: <Your Server Name>
Hope this helps