ProQuest is actually not a database, it's a platform that hosts many databases. You can access ProQuest via the A to Z Database list.
The front page of ProQuest is the Basic Search. To choose the database(s) you want to search, click on the Change Databases ➊ tab.
This will lead you to the Select Databases page (shown below), where all databases on the ProQuest platform that Tilburg University Library has access to are listed. You can select the databases you want to search here. Any one of the ProQuest databases can also be selected from the A to Z Database list.