Issue session token is not working salesforce rest api call

Issuing session token is not working in salesforce rest api call. below request is returning with

    "error": true,
    "message": "Invalid value: \"JSON body.\".",
    "code": 400403
 HttpRequest req = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http();
        string endpointUrl = '';
        req.setHeader('Content-Type', 'application/json; charset=UTF-8');  
        try {
            res = http.send(req);
            if (res.getStatusCode() == 200) {
            } else {
                System.debug('HTTP error: ' + res.getStatusCode());
        } catch(System.CalloutException e) {
            System.debug('Callout error: '+ e);

Hello @saikrishna,

Welcome to the Sendbird Community.

It looks like you’re passing an invalid JSON body to the request. The body does not take in a key value pair of user_id but rather a key value pair of expires_at with a unix timestamp. Alternatively, you can send an empty JSON body and use the default expiration date of 7 days.