Try generating your random value in the prescript tab using a function like this one:
// random generator function
function getRandomInt(min, max) {
min = Math.ceil(min);
max = Math.floor(max);
return Math.floor(Math.random() * (max - min)) + min;
}
// generate the random value
var myval = getRandomInt(0,100)
// set the value into the global variable
postman.setGlobalVariable("value",myval)
// to see it in console
console.log(myval)
Then, in your JSON body, you can go ahead with the following.
{
"Windspeed":{{value}}
}
This should work just fine.