Providing sample project on how we could use TestStep object using TFS's rest api
If you are using c# rest client then follow TestStepsSample.cs
Else if you are using JS client then follow TestStepsSample.js
Follow this blog for more details: https://blogs.msdn.microsoft.com/visualstudioalm/2016/11/02/how-to-use-test-step-using-rest-client-helper/
