Generally, there are two ways to hide a prototype’s URL. The most common way is to click on the “X” icon at the top-left of the page. This change will also be reflected in the URL and you can share this configuration.
The second way is to remove the prototype player completely when viewing the prototype and can be done by linking directly to the HTML. You can find this link by navigating to the project overview in the Axure Cloud workspace and clicking the “Share” icon. Then click on the hyperlink at the bottom of the dialog:
You can also make your pages default to the HTML by using Pretty URLs:
If you’re still seeing issues hiding your prototype player then can you clarify what you mean by pages that are opened internally?