You will need a client id and client secret.
The client should be registered in D365F&FSC at System administration -> Setup -> Azure Active Directory applications
All requests were tested using Fiddler and its Composer -> Scratchpad feature.

• Postman
• Fiddler

There are a couple of examples of how you can test your services:

API (JSON)

Make sure that empty line which dividing HTTP headers and body is empty, without space. Otherwise you will most probably get an error.

Request to get auth token:

POST https://login.windows.net/{TenantId}/oauth2/token HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Host: login.windows.net

resource=https://{D365FO-URL}&client_id={ClientId}&client_secret={ClientSecret}&grant_type=client_credentials

POST https://{aos}.cloudax.dynamics.com/api/Services/VKOServiceGroup/VKOCalcService/calc HTTP/1.1
Content-Type: application/json
Accept: application/json
Accept-Charset: UTF-8
Authorization: Bearer {AuthToken}
Host: {aos}.cloudax.dynamics.com

{
"_calcContract":
    {
         "arg1": 2,
         "arg2": 2,
         "operation": 0,
         "list": [{
             "a": 0,
         },
         {
             "a": 1,
         }],
         "subCalc": {
             "a": 5
         }
    }
}

SOAP

SoapUI can form a correct request for you using wsdl https://{aos}.cloudax.dynamics.com/soap/Services/VKOServiceGroup?wsdl
You can use SoapUI to send request, you just need to manually add Authorization header with value Bearer {AuthToken}

Example of SOAP request:

POST https://{aos}.cloudax.dynamics.com/soap/services/VKOServiceGroup HTTP/1.1
Content-Type: text/xml
SOAPAction: http://tempuri.org/Calc/calc
Authorization: Bearer {AuthToken}
Host: {aos}.cloudax.dynamics.com

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:dat="http://schemas.microsoft.com/dynamics/2013/01/datacontracts" xmlns:tem="http://tempuri.org" xmlns:dyn="http://schemas.datacontract.org/2004/07/Dynamics.AX.Application">
   <soapenv:Header>
   </soapenv:Header>
   <soapenv:Body>
      <tem:calc>
         <tem:_calcContract>
            <dyn:List>
               <dyn:SubCalc>
                  <dyn:a>1</dyn:a>
               </dyn:SubCalc>
            </dyn:List>
            <dyn:arg1>2</dyn:arg1>
            <dyn:arg2>2</dyn:arg2>
            <dyn:operation>0</dyn:operation>
            <dyn:subCalc>
               <dyn:a>1</dyn:a>
            </dyn:subCalc>
         </tem:_calcContract>
      </tem:calc>
   </soapenv:Body>
</soapenv:Envelope>

SOAPAction: http://tempuri.org/{External Name property from service}/calc

C#

http://www.k3technical.com/consume-ax7-custom-web-service-by-soap-endpoint/
http://dev.goshoom.net/en/2017/02/json-based-custom-service-with-parameters/

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