It looks like Microsoft has depricated the original Web Application Stress Test tool. In some few places various people have saved the binaries that are available for download:

Here is IBM's support site:

https://www.ibm.com/support/pages/download-and-use-microsoft-web-application-stress-test-tool-generate-data

Also, the is another tool developed by Microsoft's dev community. The tool is called WCat. Web Capacity Analysis Tool (WCAT) is a lightweight HTTP load generation tool primarily designed to measure the performance of a web server within a controlled environment. WCAT can simulate thousands of concurrent users making requests to a single web site or multiple web sites.

Web Capacity Analysis Tool could be downloaded from here:

https://www.iis.net/downloads/community/2007/05/wcat-63-x64

May I just throw in the link to Apache JMeter as an Open Source web stress test tool:

https://jmeter.apache.org/

Hope this helps!