For Turnkey on .netCore there is a page views\\turnkey\\StartupViewTemplate.htm that will be updated with :

System.IO.File.ReadAllText(path).Replace("###BASEURL###",Common.GetApplicationServerUrl());

And then returned to client while if the TurnkeyApp has not fully started yet.

The Turnkey app takes some time during startup to extract model from MDrivenServer and expand the model out to disk to be faster the next time it starts - if the model has not changed checksum.

You can also modify the displayed page by providing a page that works the same way under the name:

_AssetsTK\\Views\\EXT_OverridePages\\StartupViewTemplate_AppOverride.htm. Read more on AssetsTK

For Turnkey on Framework the StartUp page uses the built in IIS support that requires you to change the web.config file as described in this article: Documentation:Simple_URLs_and_Azure_redirect