Postman pre-request script

pre-request.js

let url_auth = pm.variables.get("keycloak_url")+"/auth/realms/" + pm.variables.get("keycloak_realm") + "/protocol/openid-connect/token";

let params = "client_id=" + pm.variables.get("keycloak_client_id")
+ "&client_secret=" + pm.variables.get("keycloak_client_secret")
+ "&grant_type=client_credentials"; 

pm.sendRequest(
{
    url: url_auth,
    method: 'POST',
    header: 'Content-Type: application/x-www-form-urlencoded',
    body: {
        mode: 'raw',
        raw: params
    }
}, (_, response) => { 
    let access_token = response.json()['access_token']
    pm.environment.set("access_token", access_token)
});