How to account for Properties.Settings.Default in ASP.NET 5

I'm referencing a third-party DLL that reads configuration from Properties.Settings.Default.DefaultHostName. How do I configure this in my ASP.NET 5 application?

In previous versions of ASP.NET this would have appeared in the web.config as

<applicationSettings>
  <DllName.Properties.Settings>
    <setting name="DefaultHostName" serializeAs="String">
      <value>Sample</value>
    </setting>
  </DllName.Properties.Settings>
<applicationSettings>