Parameterizing the REST

Hi,
I'm trying to automate a REST webservice testing. I need to send this as body username=test@mailinator.com&newPassword=yen22 It works but I would like to load the username from the spread sheet so that I can parameterize the value. I tried to create a Data source step and using get data I tried to get the data but its not working. Kindly help on this.

Thanks,
RK