How to fetch merchant data (Details on storeid)

There is no need to receive any information from MobilePay to get started calling on behalf of a merchant.
Based on a merchant VAT-number you can retrieve all needed information of a merchant, including storeId's and details of storeId.

Retreive storeId by calling GET/v10/stores with merchant VAT-number in header: x-mobilepay-merchant-vat-number 
Retrieve storeId details by calling GET/v10/stores{storeid} 
Example response:

 { "storeName": "Store 1",
  "storeId": "b3f69e80-f8dc-4d81-8f46-24a8fd8fbd9f",
  "storeStreet": "Example City",
  "storeZipCode": "1234",
  "storeCity": "Example City",
  "brandName": "The Brand",
  "merchantBrandId": "POSDK12345",
  "merchantLocationId": "10001"