How or where to get Merchant id in Startsession API?

Technical Q&A
1

As per community documentation,

  1. 1st Merchant shall onboarded with PG while selecting particular PG in config
  2. 2nd Customer will checkout on Merchant website and Shopify shall initiate StartSession API to PG provided APP URL.
  3. Once request landed to PG, PG shall validate Merchant ID for processing request. In this leg as the API request mentioned in https://shopify.dev/apps/payments/processing-a-payment doesn’t have Merchant Id so in case leg how PG shall identify Merchant weather it is onboarded with it or not ?

Request to help on urgent basis.

Thanks in advance.