400 Bad Request Error while subscribing users to topic in cloud functio

I'm trying to subscribe users to firebase topic on the cloud function. So according to official docs I tried the following code

 request({
        method: 'POST',
        url: "https://iid.googleapis.com/iid/v1:batchAdd",
        json: true,


        headers: {
            'Content-Type':'application/json',
            'Authorization':'key=server key,

        },

        body: JSON.stringify({

          "to": "/topics/name",
          "registration_tokens": ["token value"]

        })
            }, function (error, response, body){
        console.log('error:', error);
        console.log('statusCode:', response && response.statusCode);
        if (response.statusCode >= 400) {
            console.error('HTTP Error: '+response.statusCode+' - '+response.statusMessage);
          }
    });

But it's giving 400 status code instead of 200. Thank you.

via Doge