how can i pass a list of users (id , mail , password ) when invoking StartTest api with postman ?

i'm trying to launch my scripts with apis , so in my scripts i m using a list that contains data and information of my users account (id , mail , password ) , i tried to start my test with StartTest api and with a body that contains

{
"d": {
"ScenarioName": "scenario1",
"Description": "Testing rest api",
"NLWeb": false,
"Variables": "{\"env\"=\"xxxx\",\"Host\"=\"xxxxx\",
\"Users\"='[{\"id\":\"xxxx",\"password\":\"xxxx\",\"mail\":\"xxxx\"}]'}",
"Debug": false,
"TestResultName": "xxxx"
}
}

an exception was thrown :

Variable Users is not a constant

what should i do to resolve this issue ?