public class RedirectionMiddleware
{
    private readonly RequestDelegate _next;
    public RedirectionMiddleware(RequestDelegate next)
    {
        _next = next;
    }
    public async Task InvokeAsync(HttpContext httpContext)
    {

        // Redirect to login if user is not authenticated. This instruction is neccessary for JS async calls, otherwise everycall will return unauthorized without explaining why
        if (httpContext.Request.Host.Value == "domain.azurewebsites.com")
        {
            httpContext.Response.Redirect("https://yourdomain.com");
        }

        // Move forward into the pipeline
        await _next(httpContext);
    }
}
public static class RedirectionMiddlewareExtensions
{
    public static IApplicationBuilder UseRedirectionMiddleware(this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<RedirectionMiddleware>();
    }
}

Program.cs

WebApplicationBuilder builder = WebApplication.CreateBuilder(args);

builder.CreateUmbracoBuilder()
    .AddBackOffice()
    .AddWebsite()
    .AddDeliveryApi()
    .AddComposers()
    .Build();

WebApplication app = builder.Build();

await app.BootUmbracoAsync();

app.UseHttpsRedirection();

app.UseRedirectionMiddleware(); // <-- use it here

app.UseUmbraco()
    .WithMiddleware(u =>
    {
        u.UseBackOffice();
        u.UseWebsite();
    })
    .WithEndpoints(u =>
    {
        u.UseBackOfficeEndpoints();
        u.UseWebsiteEndpoints();
    });

await app.RunAsync();

app.UseRewriter(new RewriteOptions().AddRedirect("foo.azurewebsites.net(.*)$", "https://mydomain.com/$1", (int)HttpStatusCode.PermanentRedirect));