To use the 'Random Data' function, you need to choose the code and set it on the URL (for get) or POST edit text.
Eg: If your POST data are _name=JackLibertarian&email=jack@libertarian.me&text=Freedom_
Then, set your POST data using the random codes, eg: name={%first_name}&email={%email}&text={%sentence}
Doing this, you set the post data to randomly choose name, email and text. Check the code list:
{%first_name}
{%last_name}
{%full_name}
{%age}
{%email}
{%address}
{%city}
{%country}
{%password}
{%latitude}
{%longitude}
{%sentence}