How can Visual Studio 2015 WebTest run on SSL secured site without certificate?

I am using Visual Studio 2015 to run Web Performance and Load Tests on our web site. The site uses SSL throughout. The tests run correctly on our production site, but now I need to run them on our 'Staging' site. The staging site does not have a valid security certificate. When I run a webtest, I get the error: "Request failed: The request was aborted: Could not create SSL/TLS secure channel" on every request. What do I need to do to get my webtests running on the staging site?

Thank you for your help.