Its only possible to get the WSDL if the webservice is configured to deliver it. Therefor you have to specify a serviceBehavior and enable httpGetEnabled:
<serviceBehaviors>
<behavior name="BindingBehavior">
<serviceMetadata httpGetEnabled="true" />
<serviceDebug includeExceptionDetailInFaults="true" />
</behavior>
</serviceBehaviors>
In case the webservice is only accessible via https you have to enable httpsGetEnabled instead of httpGetEnabled.