I’d try something like:

string url = "https://steppschuh-json-porn-v1.p.mashape.com/search/?q=Housewife";
HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(url);
webRequest.Headers.Add("X-Mashape-Key", apiKey);

Please try to check StackOverflow for language specific questions that aren’t related to the API. That will save your and my time.

I want to create a single http request in C# that contains the search parameters and the my subscription key. how is that possible?

No, that completely depends on how you send your request. You usually have an extra field for headers. Like described here: http://unirest.io/java.html at “Creating Request”

How to I send the header with the key using Http request or url request? It goes after the address, yes?

I personally never worked with unirest in .NET, but there are plenty of guides out there. See:

https://github.com/Mashape/unirest-net
http://blog.mashape.com/unirest-for-net-c-vb-net-etc-desktop-and-windows/