<extensions>
    <behaviorExtensions>
        <add name="UserNameClientCredentials" type="Namespace.UserNameClientCredentials, MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
    </behaviorExtensions>
</extensions>

<behaviors>
    <endpointBehaviors>
        <behavior name="basicAuthenticationBehavior">
            <UserNameClientCredentials UserName="test" Password="tttttt"/>
        </behavior>
    </endpointBehaviors>
</behaviors>

<endpoint address="https://localhost:8080" binding="basicHttpBinding" bindingConfiguration="BasicHttpBinding"
          contract="MyContract" name="endpointName"
          behaviorConfiguration="basicAuthenticationBehavior"/>

Exception thrown: 'System.Configuration.ConfigurationErrorsException' in System.Configuration.dll

Additional information: The type .......  registered for extension 'UserNameClientCredentials' could not be loaded.