Hi,
I work in an IT company and want to use webpagetest to test and internal website. Should I set up a private instance for webpagetest ?
Access to this website requires login.How can I pass the login credentials through webpagetest ?
Thanks 
Yes, you’ll need a private instance with agents deployed inside the network. As far as login goes, it depends on how the login is done. Assuming it is a HTML login (not http-level) you can either script the login flow as part of the test you you can set cookies for an already-authenticated session.