How do I setup a NetBoot of El Capitan using Lion Server?

Question

I have an old MacBook running Lion Server that I want to be an El Capitan NetBoot server. I downloaded the Install El Capitan Application on a supported computer, and then copied it to the Applications folder of the server. When I launch System Image Utility to create the image for El Capitan, there is nothing under the "Sources" dropdown.

TLDR: My Mac running Lion Server, but I need it to host an El Capitan NetBoot Image. How do I go about doing this?

Thanks in advance!