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());

We also do a replace of the tag '###GOALURL###'; this will be the original URL the user tried to reach, even if this may seem trivial, and it is then returned to the client if the TurnkeyApp has not fully started yet.

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

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.

You should not underestimate the work involved in waiting for the server to start, routing through external login, and handling this for both client-side URLs (# like angular) and full URLs like DWV and Blazor.

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: Simple_URLs_and_Azure_redirect