From the information you've provided, this appears to be a normal authentication redirect that's generating the return URL based on the Laserfiche Forms Host URL configuration value.

Under Laserfiche Forms Host URL, type the fully qualified domain name of your Laserfiche Forms Server, in the format //ServerName:port/Forms. This is the URL that the Directory Server STS will redirect users to after they successfully authenticate.

I believe there may be an advanced Forms configuration setting to provide a separate "Forms DMZ URL" for those redirects. If this DMZ Forms instance should only have unauthenticated access (only used for Public forms), and there's no reason for anyone to authenticate through the DMZ route, you can modify the DMZ Forms web.config file to redirect all auth requests to a URL of your choice, like an organization homepage such as https://example.com. See attached image. 

That may not entirely remove the generated return URL parameter, but it'll likely keep it from displaying in a browser address bar.