You need to use the <serviceMetadata>
element.
<behaviors>
<serviceBehaviors>
<behavior name="metadataSupport">
<!-- Enables the IMetadataExchange endpoint in services that -->
<!-- use "metadataSupport" in their behaviorConfiguration attribute. -->
<!-- In addition, the httpGetEnabled and httpGetUrl attributes publish -->
<!-- Service metadata for retrieval by HTTP/GET at the address -->
<!-- "http://localhost:8080/SampleService?wsdl" -->
<serviceMetadata httpGetEnabled="true" httpGetUrl=""/>
</behavior>
</serviceBehaviors>
</behaviors>