How to mock FindClient search queries in Optimizely?

Hello,

I am currently working on mocking optimizely search client interface IClient for my unit tests that are using xunit. 
Specfically this statement right here:

  var cmsContent = _client.Search<IContent>()
    .For(query)
    .GetResult();

I have tried using Moq, and NSubstitute but with I cant seem to mock the statement above.

Anyone have any idea how to mock the above statement?

Any help is appreciated.