I wanted to set my starting page to /Members/Index.
When I was using MVC, I configured it as following:
app.UseMvc(routes => {
Add the following to your ConfigurationServices function in Startup.cs
services.AddMvc().AddRazorPagesOptions(options => { options.Conventions.AddPageRoute("/Members/Index", ""); });
If you have another index page you'll probably need to delete or rename it.