ProQuest isn't indexed in EBSCO Discovery Service, however they have a way to set up a widget which, when clicked, duplicates a search in ProQuest with the correct holdings selected.
https://gssapps.ebscohost.com/llbb/download/ProQuest_Widget.pdf
We followed the steps in the instructions but because we want users to be able to search without going through EZProxy, but be proxied when they attempt to exit to ProQuest, it didn't work. EBSCO are now working on a technical fix.