There are a few options available for you on Windows IoT Core
If you're building an application using Node.js there's Mosca (http://www.mosca.io)
If you're building an application using Python, you can check out hbmqtt(https://github.com/beerfactory/hbmqtt)
Of course for .NET application,s you can search Nuget. GnattMQ(www.nuget.org/packages/GnatMQ) seems to be pretty popular .NET library