This took me way to much time to figure out since there is a ton of old information on the internet. I wanted to change the default behavior when people are logging in to my ASP.NET Core website using Azure Active Directory (or Microsoft Identity Platform). After some searching I figured out how to change this setting.
You have to add the following piece of code to the ConfigureService method in your Startup.cs
services.Configure<OpenIdConnectOptions>(AzureADDefaults.OpenIdScheme, options =>
options.Events = new OpenIdConnectEvents
OnRedirectToIdentityProvider = async context =>
context.ProtocolMessage.DomainHint = "demo.hoekstraonline.net";
Same trick works for Login_Hint
Hope this saves me some time next time I am looking for this information.