Now this may be deprecated, but this is how I install and work - downvote and let me know if it is wrong, and I will remove it. I got it on the blog / MSDN page, but I can no longer find it.
First you need to create a TFVC team project (it doesn't matter if you use it again).
In $/MyTFVC/BuildProcessTemplate/CustomActivities/
Checking the following files from xunit.net:

Now in VS, click the BUILD button, Manage Build Controllers .... Select "Hosted Build Controller (Hosted)" and click "Properties ...".
Enter the path that you specified in the DLL in the "Version control path to custom assemblies" field:

You must be kind.
source share