MerchantId is a unique identifier of a merchant in our system. After you retrieve an access token from OpenID flow use the following endpoint to retrieve your MerchantId.

GET /api/v1/merchants/me

Response

HTTP 200 OK

{"MerchantId":"e3465d75-8fd4-4ea4-8881-304c464f1d24"}

Read more here

Afterwards you should get an invoice issuer 

GET /api/v1/merchants/{merchantId}/invoiceissuer