I have no idea about what your request suppose to do here. If you need to pass JSON then it should have RecContentType=application/json"
Please refer and take a look at very good post http://h30499.www3.hp.com/t5/HP-LoadRunner-and-Performance/How-to-use-JavaScript-in-your-HP-LoadRunner-scripts/ba-p/6197321
I hope it will be of great help.
Thanks!
Sanjay