Well, you must set this property otherwise it won’t work.
In previous version of SonarQube we default to https://api.github.com if this property was not set, but we remove that because we don’t (and never have) officially supported Github.com with SonarQube.
So you have to set the property to https://api.github.com instead of a GitHub Enterprise url, but be aware that it’s not (yet) officially supported (see previous link to understand why).
HTH
Benoit