How to get the page scope id of Facebook when user

Question

Before facebook last update I was using Facebook Send message API by passing user reference. I was getting this information by checking the checkbox during checkout in my store (Shopify) using Facebook.

Now the Facebook Send message API no longer returns recipient_id in the response for message sends that use recipient.user_ref or recipient.phone_number to identify the message recipient.

Now they are using the Page scope id syntax is

 recipient": {
"id": "<PSID>"
},

i am using this API

How can I get this page scope id?