Instagram Subscription API get response hub.challenge

I'm reading the Instagram API docs https://www.instagram.com/developer/subscriptions/

And I understand the necessary steps

but on this line

In order to verify the subscription, your server must 
respond to the GET request with the hub.challenge parameter only:

After Instagram sends a POST request to my callback, when they state "server must respond to GET request" does that mean when they call my GET callback, I instantly send a post request within the GET callback. Inside this post request I send the hub.challenge to " https://api.instagram.com/v1/subscriptions/"

so something like

getInstagramSubscription(req,res){
     //Grab hub.challenge and send post request to https://api.instagram.com/v1/subscriptions/

},

I tried using instagram-node-lib

 getInstagramSubscription(req,res){

     var handshake =  Instagram.subscriptions.handshake(req, res);

},

but once I deploy on heroku with this curl request

 curl -F 'client_id=client_id' \
 -F 'client_secret=secret' \
 -F 'object=username' \
 -F 'aspect=media' \
 -F 'verify_token=myVerifyToken' \
 -F 'callback_url=callback_url/subscriptions' \
 https://api.instagram.com/v1/subscriptions/

I'm still getting a Challenge verification failed

via testinggnitset ser