Correct value of merchantIdentifier when using Web Merchant Registration API

Hi!

We recently signed up and activated the Web Merchant Registration API and registered a few of our merchants.

We registered them as:

{
    "domainNames" : ["X"],
    "partnerMerchantName" : "Name",
    "partnerInternalMerchantIdentifier": "MID-###",
    "encryptTo" : "platformintegrator.X",
    "merchantUrl": "https://www.x"
}

When we make the payment with startsession, we are getting an error 400:

Payment Services Exception merchantId=platformintegrator.x not registered for domain=x

I suspect that this has to do with the value of merchantIdentifier we are setting in the startsession.

What should it be? MID-###? The encryptTo hash of platform? platformintegrator.x?

Also, what's the meaning of the property on the sub-merchant I get from the GET API call delegatedCommerce: {enabled=True}?