Add a new B2C letter

curl --location --request POST '/send/letter' \
--header 'Content-Type: application/json' \
--data-raw '{
    "Letter": {
        "Channel": "PRINT",
        "ChannelSettings": {
            "Digipost": {
                "SenderId": "1234567"
            },
            "Email": {
                "SenderName": "Company AS",
                "SenderEmailAddress": "no-reply@company.com",
                "Subject": "Information from Company AS"
            },
            "Kivra": {
                "SenderId": "1234567"
            }
        },
        "ExternalUrl": "https://amili.no",
        "FreeText": "PDF_attachment_encoded_in_Base64",
        "LayoutID": "",
        "Note": "",
        "Identifier": "unique_invoice_identifier",
        "LanguageID": "NO",
        "Invoicee": {
            "OrganizationNumber": "",
            "CustomerNo": "6854984",
            "NationalId": "010572xxxxx",
            "Name": "Thomas Hansen",
            "Address1": "Drammensveien 30",
            "Address2": "Drammensveien 302",
            "Address3": "Drammensveien 303",
            "City": "Oslo",
            "PostCode": "0620",
            "CountryCode": "NO",
            "Email": "thomash@gmail.com",
            "Mobile": "4799653241",
            "Reference": "Ref00001"
        },
        "Invoicer": {
            "CompanyRegNo": "98741562",
            "Name": "Utleier AS",
            "Address1": "Kongeveien 1",
            "City": "Oslo",
            "PostCode": "0580",
            "CountryCode": "NO",
            "Email": "contact@company.com",
            "Phone": "4722556699",
            "WebAddress": "www.company.com"
        },
        "OtherReferences": [
            {
                "Type": "urn:no:ccn:einvoice:latepayment:fee",
                "Value": "70.00"
            }
        ]
    },
    "Subscription": {
        "SubscriptionId": "DYubVXsH5pkH5rNXMJm9zjNQWSjQ1wYr",
        "StorageId": "267"
    }
}'