Access to Azure Communication service to send SMS via REST api failed: "Our services aren't available right now"

The state is:

1. "Communication Service" subscription to send SMS
   "Try SMS" on Azure portal works OK
2. There are: {end point} and "primary key"
3. Simple C# windows desktop app with .NET SDK runs OK and
   "SmsClient smsClient = new SmsClient(connectionString)"
   send SMS successfully
4. Azure CLI running on Windows computer send SMS successfully with command line:
   az communication sms send --sender "+1888ddddddd" --recipient "+1dddddddddd" --message "Hello world" --connection-string "endpoint=https://biopac-sms-otp.unitedstates.communication.azure.com/;accesskey=..."

The goal:

1. to develop C++ app that utilize Azure Communication Service via REST API by sending POST request to Azure service because there is no Azure SDK for C++
   https://learn.microsoft.com/en-us/rest/api/communication/dataplane/sms/send?view=rest-communication-dataplane-2021-03-07
2. Running POST request from C++ windows app to the same {endpoint}+"primarykey" as C# example returns error.
3. Tried to send POST request via PostMan utility and got error message:
   "<h2>Our services aren't available right now</h2><p>We're working to restore all services as soon as possible. Please check back soon.</p>0u/LuZgAAAADZpgZDPtx2Saux4t9VGuIIU04xRURHRTMxMDgARWRnZQ=="

But this is not true. C# windows app can send SMS via Azure Communication service

1. POST request is:
   https://biopac-sms-otp.unitedstates.communication.azure.com/;accesskey=.../sms?api-version=2021-03-07
   Request's Body:
   { "from":"+1888ddddddd", "smsRecipients": [ { "to": "+1dddddddddd" }], "message":"Hello world", "smsSendOptions": { "enableDeliveryReport": true, "tag": "testSmsRequest"

}

}

What is wrong about "POST" request to Azure service?

Thanks,

Mikhail