apim-grid-validation.xml

<when condition="@(context.Variables.GetValueOrDefault<bool>("isEventGridSubscriptionValidation"))">
    <return-response>
        <set-status code="200" reason="OK" />
        <set-body>@{
              var events = context.Request.Body.As<string>(); 
              JArray a = JArray.Parse(events);
              var eventGridData = a.First["data"];
              var validationCode = eventGridData["validationCode"];
              var jOutput =
                 new JObject(
                    new JProperty("validationResponse", validationCode)
                 );
            return jOutput.ToString();
        }</set-body>
    </return-response>
</when>