APIM_RelayToken_Policy.xml

<policies>
    <inbound>
        <set-backend-service base-url="*** add your wcf relay address here ***" />
        <cache-lookup-value key="@("relaytoken")" variable-name="relaytoken" />
        <choose>
            <when condition="@(!context.Variables.ContainsKey("relaytoken"))">
                <set-variable name="resourceUri" value="@(context.Request.Url.ToString())" />
                <!-- an accessKey entry must exist in NameValue -->
                <set-variable name="accessKey" value="{{accessKey}}" />
                <!-- an keyName entry must exist in NameValue -->
                <set-variable name="keyName" value="{{accessKeyName}}" />
                <set-variable name="relaytoken" value="@{
                    TimeSpan sinceEpoch = DateTime.UtcNow - new DateTime(1970, 1, 1);
                    string expiry =  Convert.ToString((int)sinceEpoch.TotalSeconds + 3600);
                    string resourceUri = (string)context.Variables["resourceUri"];
                    string stringToSign = Uri.EscapeDataString (resourceUri) + "\n" + expiry;
                    HMACSHA256 hmac = new HMACSHA256(Encoding.UTF8.GetBytes((string)context.Variables["accessKey"]));
                    string signature = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(stringToSign)));
                    string sasToken = String.Format("SharedAccessSignature sr={0}&sig={1}&se={2}&skn={3}",
                    Uri.EscapeDataString(resourceUri), Uri.EscapeDataString(signature), expiry, context.Variables["keyName"]);
                    return sasToken;
                    }" />
                    <cache-store-value key="relaytoken" value="@((string)context.Variables["relaytoken"])" duration="10" />
            </when>
        </choose>
        <set-body template="liquid">
            <!-- set your body transformation here -->
        </set-body>
        <set-header name="ServiceBusAuthorization" exists-action="override">
            <value>@((string)context.Variables["relaytoken"])</value>
        </set-header>
        <set-header name="Content-Type" exists-action="override">
            <value>application/xml</value>
        </set-header>
        <base />
    </inbound>
    <backend>
        <base />
    </backend>
    <outbound>
        <set-body template="liquid">
            <!-- set your body transformation here -->
        </set-body>
        <set-header name="Content-Type" exists-action="override">
            <value>application/json</value>
        </set-header>
    </outbound>
    <on-error>
        <base />
    </on-error>
</policies>